Level select
Hold [Command] + [Option] at the main menu and choose the "New Game" selection.
All weapons
Hold [Command] + [Shift] and type 0wpn.
Invincibility
Hold [Shift] + [Space] and type prime.
Full health
Hold [Command] + [Shift] and type iv.
Full ammunition
Hold [Command] + [Shift] then type a number that corresponds to a weapon, followed by wpn.