airsoft + vex robot

ok i have avex robotics kit right. and i want to put a airsoft gun on it. the robot now is like a 4 weeled car. and i have a place to mount and stuff but i dont know how to make it fire by remote control of any kind. First i tried the little controller tht my sercos plugg in thinking tht when i press the accelerator it would send a signal to the servo at tht time but its not like tht. Its is wierd it has a constant voltage and the servo sorts out when it gets fm signal to move. Also is there a way i can use a wirless xbox controller the old ones?? OR ANY OTHER IDEAS PLEASE HELP?!!