Well I don't know how to use a BlackBerry as a modem on a mac... I do know it has to be over USB though; as even on a PC you can't use it over Bluetooth.
But I will warn you about one thing; You said you have Rogers "unlimited". Thats not truely unlimited im afraid; many people have been complaining as there is a cap at some point... most people came up with 25MB being the cap before they basically say "your abusing the unlimited or whatever, and we will now charge you extra". So be careful with that, especaialy when thethering.... you could rack up some serious charges really quick. http://blackberryforums.com/showthread.php?t=26080
<< That thread has a discussion on what I'm talking about.