[red67]

if you just want music to come out of a p.a. from itunes on your mac then you can use the headphone jack out to line-in on an amp no problem,
all macintosh towers have line outs at the back and headphone jacks at the front. either will go into an amp as above.
don't know if the latest imacs have line outs but headphone jack will be fine, your mini should have a headphone jack and that'll be fine too.
you can use an ipod also, best to bring the usb power plug, so battery doesnt die if you do.