As usual, the level is unbeatable, but you can play through it. Also, you can edit the level in the level editor.
To use the tank, you first must kill the driver, then jump onto the top to get in. Jump again to get out of the tank. While in the tank, press space bar to fire the cannon.
This level is considerably more...I guess...linear than the other levels(Not that the other ones have been that non-linear). I did however manage to stick a secret passage into the level, so see if you can get to it.