To get mine working, I opened Steam, Selected 'Play Games' Right-click on HL2, then properties Select Launch options Enter [-console] (without the [ & ]) Then once in game, tilde to toggle the console. In console I typed sv_cheats 1 It works fine.
Did you open up your developer's console? If not, then go to options, go to keyboard, and click the advanced button. Once you got that up check off the enable developer console or whatever, and if you did that use all the cheats that everyone posted up.