In the game itself... hit TAB to enter the config menu for mame. Select the one that says "INPUT (this game). Hit enter on the item you want to assign, click on the button. If you ever screw up, you can simply hit enter and then the ESC button to clear it. :-)
For Robotron specifically, there are 4 inputs that say: P1 Right/up etc. That's the one that fires (right joystick). The other 4 inputs say: P1 Left/up etc. Those are for the movement (left joystick).
When playing the game, if you choose 2 players, the players switch spaces when it's their turn.
Hope this helps.
DeLuSioNaL29