Just started a Warlock, but my Imp doesn't seem to do anything but follow me or stay. The only way I can get him to attack is by manually clicking his attack when targeting an enemy. I don't know if this is normal or not, but I get the impression that it isn't. He won't defend or attack when I use the attack command, and I'm worried the same will be true for future summons.
Have you tried to change imps stance? It's probably on Passive in your case. Change it to the Assist ( ctrl + 8) and try to attack the dummy or whatever you like. Imp should start firing firebolts if ofcourse you right clicked on firebolt and now it's all shiny around the firebolt icon.