Moves are not triggered through combinations of buttons in thsi game. To select an attack, press a number on your keyboard (1-9) to open the attacks menu, and it will show you a picture of the attack and what number to choose for that attack. After you've selected, you use your fire button (Mouse button 1 or Enter by default) to use it.
BTW, I don't know about using it on CS retail, I think you need the full version of HL.