At the main menu screen of the demo enter the "Game Options" and Enable the Console. From there, push your tilde (~) button to bring down the console and type the following:

seta thereisacow "1337"
followed by
spdevmap eldaba

This will start the demo level with devmap and cheats enabled. This allows you to use any of the commands in game without restriction. When in game, bring down the console and enter any of the commands from below to activate the 'cheat'.