Gaining access to the Developer Console Commands will give you a lot of new options you wont find in the main options menu.
- Startup the Alien Swarm.
- Click through to Options > Keyboard/Mouse
- Check "Enable Developer Console".
- Save and close out of all of that.
- Press the ` key (usually in the top left of your keyboard).
- Enter desired commands.
Once you do this you'll be able to enter in any commands you wish from now on. They will not be saved if you close down your game though. To save them you'll want to setup Stored Console Commands With Autoexec.