Use WASD to move and press the key you want for the weapon. Example, press 1 for the pistol and 3 for the rocket launcher. The weapons are a little glitchy and the SMG doesn't work. Press K to reload all weapons. There are a lot of extra variables on the screen for me to use since the game is incomplete.
Also try this game: http://scratch.mit.edu/projects/27086135/ as well as this: http://scratch.mit.edu/projects/20904662/ Shared from Scratch 2 Offline Editor