If you google "bmw z4 bluetooth pairing" you will find step by step instructions and even some videos on going through the process.
Now, one big question: Does your Z4 have bluetooth installed?
After posting my first reply, I came home and dug up a 2007 Z4 brochure. It appeared that bluetooth was optional and was included if you got BMW Assist or the Premium Package. Does your vehicle have an SOS button under a clear plastic cover at the very rear of your console (that's BMW Assist)? If not, you may not have bluetooth and the whole discussion is moot.