This forum post at BlackBerryForums.com has all of the details necessary to set up tethering for a BlackBerry Bold on Mac OS X.
I set up tethering quite a while ago. I then updated my Bold firmware not realizing that it would break tethering. After some digging, I found that the solution is to install the BlackBerry Bold PPPD Replacement. This solution is mentioned on the last page of the same BlackBerryForums.com post and also on this CrackBerry.com forum post.
Now tethering works correctly again, and I still have the benefits of the upgraded firmware on my Bold.
Update: BlackBerryForums.com has removed the old post for tethering on Mac OS X. This post appears to be a suitable replacement, but I have not tried these exact instructions.
Hi.. the post for Mac is no longer there.. do you know where else I can get the instruction?
@Shine, thank you for alerting me to this. I have posted a new link to what appears to be updated instructions. I hope that helps.