First place to look is on the stereo itself.
turn it on, then hit mode a few times.
if you only have two modes (CD and Radio) then you neither have a cd changer, or an ipod.
if you have cd,radio and CD-1, then you could have iPod...
In this case, have a little lookie in the glovebox.
if it has the standard bmw ipod connector, then there'll be an ipod cable in there.
also look in the center compartment between the seats.. you may have an intravee etc.
If you have a CD changer, try sticking a cd in it and see if it plays.
if it does play, then I'm 99.99% sure you haven't got an iPod connection.
if it doesn't play / doesn't work, it's likely that it has been unplugged, and this usually only occurs if the owner is going to plug in an iPod.
if this isn't the case, you can buy an iPod connector from BMW for about £140.
DON'T pay for them to fit it tho, as it's piss easy to do yourself!
Part numbers:
iPod Adapter for DSP stereos: 65410409611
iPod Adapter for non-DSP stereo: 65410409610
info:
http://www.realoem.com/bmw/showparts.do?model=BT52&mospid=47797&btnr=65_1645&hg=65&fg=95
hope that's helpfull