Hi Eric,
I moved your post to the REW forum, because I think it's the interface between the iMac, or the usage of REW with the iMac that you will be concerned about. The BFD itself doesn't really require any interaction at all with the iMac. Yes, the iMac/PC can connect to the BFD via MIDI to more quickly download filters, but it is not necessary at all (it's not really that hard at all to set the filters by hand via the front panel of the BFD).
Now, I've no used a Mac with REW, but I believe that they all work together (others have had success, and there may be information on older threads via the "Search" feature). You will need Java installed, you'll need a "line in" and "line out" on your iMac ("mic in" will not work), you'll need a bunch of cables and connectors and you'll need either an SPL meter or a mic and appropriate preamp. That's generally the stuff you need to get REW working.
I think you may have said that you have another means to measure room frequency response, and if so, that's fine. The only downside there is that that tool won't calculate the filter settings necessary to flatten your response, and it won't interface with the BFD via MIDI. Still, you can do your measurement with your tool, then set filters by hand based on those readings, then measure again with your tool to determine effectiveness. Repeat until you are happy.
If you intend to go with REW, please have a look at the well-written user's manual available at the top of every Home Theater Shack page. It looks like this:
Attachment 5230
I don't know if you've read it yet, but there's also a BFD manual button at the top of every HTS page that looks like this:
These two manuals are really good reading to get one on the right track!
Good luck!