Hi Bluto,
I have the manual for this game. In the manual, it says to use the directional arrow keys to designate the attacker (Largo, Simon, or Joy) and then validate your choice by hitting the spacebar. Choose the enemy you wish to attack in the same way, and lastly choose the type of attack you want to make. You can cancel your choice at any time by hitting the backspace key.
Let us know if this helps.