I asked Colin if he would go ahead and add native support for the X-plorer for the next release of Pref360, and he has now done exactly that in the just released version 0.05. Furthermore, it works great with Frets on Fire (look here for the Mac version); just install Pref360 and then go into the Frets on Fire button setup and you should be able to assign the fret and strum buttons (the whammy bar is not yet supported in Frets on Fire). Now you have access access to all the fan created songs for Frets on Fire on your Mac.
Coincidentally, MacRecon recently reported that you could use the X-plorer with USB Overdrive ($20). However, Pref360 is donationware, and it works with Frets on Fire. So unless you have some other need for USB Overdrive I'd suggest going with Pref360, which is a free download from Tattiebogle.
Thanks gortman (mac recon)!
Update: fixed preposition. For those of you having trouble getting Frets on Fire to run, I would suggest trying out one of the older builds. Version 1.1.324 from back in November is running fine for me on an Intel Mac. Look here and scroll down.