You need the ipod adapter. It removes the cd changer from your glove compartment and installs there. Easy job but I believe it was about two hundred dollars for the part...it been a while.
Do a search. Im sure there is a dyi for the ipod adapter install.
2005 BMW 645ci - DPE 3pc Forged Rims, Coilovers, Britalman full cat-back exhaust system, , K&N Air Filter, LUMMA Roof & Trunk Spoiler.