Moved to the 8100 section.
Install the latest version of the Mac Desktop manager.
-Start Blackberry Desktop Manager
-In the Window Menu open the log window
-Connect your cable to your Blackberry first and then connect to the other end to your mac
-Now you should see some log lines starting to appear until they repeat showing "Throwing Unsupported Device....." It will loop about 20 to 25 times, then, the Desktop Manager will recognize that it needs to upload the OS on the device and suddenly you will get the window to choose which OS to load. Make sure the battery is in the blackberry otherwise it won't be able to recognize the device PIN.
That should allow you to do it. If not, see below:
Set up a Windows instance using Bootcamp, Parallels or VMWare and install windows and do it that way. Or find a friend with a PC who will let you install DM and the OS.
Maybe this will help as well: Blackberry Reload Software 507 Mac | BlackBerryHandset.com Reload Software: 507 error - BlackBerry Forums at CrackBerry.com