You need to get a Firewire cable. Hook both macs
together. Now, take the Macbook and reboot it... as
soon as it resets hold down the "T" key. This will put
the MacBook in target disk mode. On your iMac you will
see an Icon of a Firewire HD -- this is your Macbook's HD on
your iMac -- navigate to your iTunes folder (this will be in
the "Macintosh HD/Users/USERNAME/Music/
Just grab the contents of it and put it in your iMacs HD in
the same location... and that should do it.
REMEMBER to drag the Firewire/Macbook HD in your trash to
eject it before turning off your Macbook.