0, download bb modem script here, http://www.fibble.org/archives/2007/10/updated-modem-s.html
and put it to /Library/Modem scripts/
1. pair your bb with your mb, at the last step, use the info as below to setup the phone modem.
2. create new bluetooth connection in network pannel of system configuration. Fill the blanks on the right with the below info again.
3. go to advanced, Choose the vonder as RIM, and Model as 8800, Dial mode: ignore dial tone while dialing,
T-Mobile:
phone number: wap.voicestream.com
username: guest
password: guest
At&t:
Username: WAP@CINGULARGPRS.COM
Password: CINGULAR1
Phone Number: wap.cingular
refer to this link for more details instruction, http://theappleblog.com/2009/06/24/how-to-tether-your-blackberry-to-your-mac/