If you're at a level that you just can't beat, press CTRL and the PLUS (+) key. Instant win!
Hidden Camp (Level 20)
To play the secret level "Limbs, Heads and Smoking Craters" get all the units you can and leave level 20 ("Murder of Crows") through the northeast corner.
Level Select
When selecting NEW GAME hold down the Left Shift button until the levels come up. You will now be able to choose any level.
Secret Level (Level 3)
To play the secret level ("A Long Awaited Drinking Party"), start a 1-Player game and advance to the third level, "Down a Broken Path". Prior to completing the third level, kill the three frogs on the tiny oasis. (They are located in the Northwest corner of the map.)