For the following cheats to work, you must be facing north when you type them in:
Get access to all the items
You must first enter your inventory. When you've done that, go to the mini-medipak and hold down A + L + T + G + R together.
Skip the level
Once again, enter your inventory, and enter the Load Game section. Now hold down L + I + F + T together.
Invulnerability
Go to the inventory again, but this time go to the flares and press F + I + R + E together.
Get access to all of the weapons
Once in the inventory, go to to the large medipak and hold down C + T + R + L together.