Please see this thread.

http://www.xoutpost.com/bmw-sav-foru...-e70-4-8i.html

there are two star bolts you need to remove that attach the alternator to the engine block. Not sure you have already removed them
Once those are removed it should slide right out. At least it did for me.

My big issue was fitting in the new one. Lot's of cursing and same thoughts of C4.
My issue was on the new alternator the bushing was pushed out and it would not allow the alternator to slide back into place. Just FYI if you hit an issue getting the alternator back in.