I just finished shooting with the AF100 on an SDXC card. My mac isn't even acknowledging the card. My sdhc cards are reading just fine. I have tried the card reader as well as the camera for log and transfer.
In theory anyway, the difference between SDXC and SDHC is only software. There are some very small differences in the SD card's ROM, and the standard also requires the use of exFAT rather than FAT32 as a file system (>4GB files! Hurray!).
Most supported computer OSs have been updated to support SDXC within the last year. Apple's usually kind of slow at adopting new technology other than the stuff they're pushing (eg, brand spankin' new Thunderbolt port in the new Macs, but no USB 3.0 or Blu-ray from Apple yet), but they have added support for SDXC. In places. Here's an article from last fall on the subject:
It should just be an OS thing... either your OS had a driver that supports SDXC or it doesn't.
FCP or any other application is working at a higher level.. it shouldn't care if the card has FAT32, exFAT, HPFS+, ext4, or NTFS as long as the driver's there to support it. Assuming FCP works with SDHC, there's virtually no chance it would fail with SDXC, if the OS otherwise supports SDXC.
Should be easy to check, anyway. I'm not a Mac user, but I assume that the OS normally auto-mounts a memory card when you put it in the slot, and there's a way to get the Mac Finder looking at that memory card. When you can examine the exFAT card this way, but can't see it in FCP6, that's the time to start thinking the problem is in FCP6. Otherwise, I would make sure I had all of the very latest OS updates. As that link I posted suggests, it seems kind of random whether SDXC is supported on any given Mac. But definitely not on older versions of the OS.