Unlimited Mana:
Complete all 56 missions in the episodes.

Unlocking the hidden level, Dark Origin:
To unlock Dark Origin, you must complete Zerg Brood War mission 9, "The Reckoning", with 5:00 or more remaining on the clock.

Unlockable cheats:
In the PC version of Starcraft, the only thing you needed to do to cheat was to type in a little phrase, and you were then able to use the cheat for the rest of a mission without incident.

Things are a little different in Starcraft 64. Cheats are not available from the start, but are instead unlocked through completing special tasks. You're also not allowed to use a cheat to complete a mission. If you try this, you will see "Cheated!" after completing the mission instead of "Victory!", and will be forced to restart the mission over again.

With that in mind, here are the eight unlockable cheats in Starcraft 64.

All Upgrades - In "The Jacobs Installation", MOVE Raynor north through the door, then right. You get the cheat after he passes by an orange civilian. DO NOT PRESS B.

All Research - In "The Amerigo", move Kerrigan to the southwest corner of the pen with all of the critters in it.

Open Tech Tree - In "Into the Darkness", go southeast until you see stairs, then southwest into the hall. When you're told to turn around, go back a ways, then return.

Special Cheat (Invincibility) - Clear every mission in the first three episodes.

Fog of War - Off In "Legacy of the Xel'naga", touch the white flag on the northeast island with a ground unit.

Give Me Cash/Gas - In "Patriot's Blood", locate the Ursadon on the northern part of the map. When it asks for a hug, surround it with units and press B.

Mega Build - In "Vile Disruption", after saving the second Hive, follow the eastern wall until you see a disc.

Infinite Mana - Clear every mission in all six episodes. Note that this does NOT include the secret levels Dark Origin or Resurrection IV.

Unlocking the secret two player level, Resurrection IV:
To unlock the exclusive two player level in Starcraft 64, you must literally do everything else first. This means clearing all the normal campaign missions, Origin, and unlocking all the cheats. When all this is completed, Resurrection IV will be unlocked and ready for action.