How to unlock Sub-Zero and Scorpion for VS mode:
After Sub-Zero freezes the castledoor in the Wastelands break it and enter. Exit back out and jump up on the dragon statue to the left to find the ying yang containing Sub-Zero for VS. In foundation follow the pathway downstairs to the left of the save station.

Two archers will bust through the wall. Throw one at the wall to the left of the door way and it will bust open revealing a secret room full of lava and a ying yang on a ledge. Grab one of the archers and throw them in the lava. Then double jump off the guards back to claim the Scorpion ying yang for VS.

Unlock Mortal Kombat II (Arcade Game):
Complete all 5 Smoke Missions

Unlock Scorpion and Sub-Zero for single play missions:
Scorpion - Beat single player as Liu Kang
Sub Zero - beat single player as Kung Lao

Unlock Baraka:
In the soul tomb portal, got to the area where you have to wall climb. You will see 2 paths. One leads left, the other right. The right path has a statue. Use fist of ruin and break it. Keep going and use the long jump ability across the gap and you will get him.

Unlockable characters:
To unlock the following characters for versus mode, simply find their "koin" (the red floating circle with the MK dragon on it) and touch it by simply running/jumping into it.

Kitana - At Evil Monastary, find area w/ two statues & portal behind it. Jump on broken pillar at left, then onto the platform. Go left some more to find koin.

Reptile - Living Forest. Area w/ large waterfall. Take clay soldier, break his shield, throw him at stone pillar on right side. Go to previous area to find koin

Jonny Cage - Wu-Shi Academy. In area w/ fire jump onto structure next to 1st wheel, then jump into alcove on right. The koin is sort of hidden, but it's there.

Baraka - Soul Tombs. After iron maiden area climb up top, break evil statue, and jump across the gaps to get the koin (next to guardrail at the upper exit.)

Unlock MK2:
In the Living Forest, there is an area with four statues. Two statues come to life, send one of those statues through the blue wall where they stand and you'll discover a secret entrance. This leads you to Smoke. Complete his 5 missions to unlock MK2.

Unlock Kitana:
In the room where you get the multality there is a broken pillar in the far right corner....jump on top of it, and then jump on top of the ceiling of the multality room...Kitana's icon is on the far left side.

Unlock Johnny Cage:
In the Wu Shi academy area where you have to put out the fires, jmup onto the house next to the test your might and then jump into the tower to get Cage's icon.

Unlock Reptile:
In the Living Woods area that has the entrance to smoke, there is a stone pillar on the far side of the stream. Break one of the stone warriors shields and knock them into the pillar to break it. After it is broken, backtrack to the waterfall u had to jump past and youll notice the icon in the cave where the waterfall was...thats reptile.(the easiest way to break the pillar is to do an air throw or air strike into it.)

Unlock both Scorpion and Sub-Zero for Single Player and Ko-op:
As an alternative to beating single player with Liu Kang and Kung Lao separately in Single Player, beating Ko-op mode will earn you both Scorpion and Sub-Zero.

Free Experience:
Tired of having to constantly build high level combos to get the experience points for that new move? Keep your ears open and pay attention. Whenever you reach a multiple of 10 in your combo meter, notice that the announcer will say something complimentary (ex. "Outstanding," "Superb," etc.) However, at random times, you will hear the legendary "TOASTY" shout instead of the announcer. As soon as you hear the yell, press START; if done properly, the screen will flash "TOASTY! 1000 Pts." and you will receive an automatic 1000 Experience Points for free!

Fight Ermac:
To fight Ermac in the game you need the swing and wall run ability. simply go to the warrior shrine then use the wall run ability on the wall to your left. Then use the swing ability and swing across the poles then interact with the shrine and it will open the doors. Go across the bridge and u will find statue of the greatest fighters. Interact with the last statue on the left about three to four times and it will eventually shake and crack open. then You will begin to fight Ermac who is very hard because he throws you around the whole shrine.

Fight Mileena:
Enter the Living Forest and continue a little bit on until you reach the first save statue. Walk a bit towards the screen and you should see a branch above you. You need the swing ability from the Dead Pool to swing to another branch and then on to a ledge. Jump up and grab the ladder. Climb up and you'll reach a hidden area. Continue on crossing the bridges and fighting enemies along the way if you wish and eventually you'll reach an area with a portal in the back. A cut-scene will begin and you will face Mileena with two other Masked Guards. Take out the guards before you concentrate on Mileena. You get some experience for killing her.

Free Health at Save Point in Ko-op Mode:
For free health in Ko-op mode, activate the save point multiple times. Each time you activate the save point, your health will increase. You don't have to save each time you activate it for this to work. You can get almost full health from doing this many times.

Infinite experience in Ko-op mode:
In Ko-op mode, go to the Living Forest and go to an area with clay golems. The best area is probably at the door in the area with the waterfall (right near the door that leads to smoke). Kill or knock away one of the golems and try to leave the other one unharmed. Now, without destroying the unharmed ones shield, trap it against a wall and have both players keep tapping an attack button (X works the best). The golems shield will not break and it will not lose any health. As long as both players are attacking the Ko-op points will keep going up. After every 10 ko-op points both players get a larger and larger experience boost. Just keep attacking and your experience will never stop going up.

Infinite Experience at Wu Shi Academy - Single Player:
In the Wu Shi Academy stage (in single player mode) play to the part where Cage opens the armory for you. Instead of going and getting the bladed weapon and the health increase, stay in the area with Cage. This section infinitely spawns Tarkata in groups of three. You can combo one, Fatality another, and finish off the third with Cage. You can build up a decent combo and every Tarkata you Fatality gives you over 500 experience. You can max out a character fairly quickly, and this is only the second stage of the game!