Using T-Mobile, so for the most part followed Clayton Long’s instructions:
Tethering Your Mac to a Blackberry (Bold 9700) on T-Mobile
I got the the proper “modem” scripts by installing the BlackBerry Desktop Manager for Mac rather than from the standalone download Clayton provides.
For “Account Name” and “Password”, used “gprs” for both instead of phone number, etc. Suggested “Telephone Number” of “*99#” and APN of “epc.tmobile.com” worked for me, but be sure to double-check your APN in your BlackBerry’s Options screen.