First, play Five until you have the lab area.

Second, buy the door that has claymores, then go behind the claymores where there is a window.

Thrid, break the bottom two right first, then left.

Fourth, just fix one barrier. It has to be the very bottom board, then jump on it.

Fifth, fix the next one but time it right - just before it fixes you jump and you should land on it.

Sixth, jump on the door ledge and then jump and crouch. Yyou should land at the very top, then move up and you're in the zombie spawn.

Warning: Once you're in there's no way of getting out (best to play on split screen).