Hello, I seem to be having a similar issue.
I just purchased the USB MicroSD Reader - MicroSDXC Reader more specifically. I paired it with a 64GB MicroSDXC card and it is not recognized in my MacBook Air 11" laptop - on Mountain Lion.
The combination works fine on a Windows 7 laptop. The card is formatted in exFAT. I also tried formatting it to FAT32. Still not recognized in my MacBook Air.
I also tried a 32GB MicroSDHC card and that is recognized just fine.
The device seems to work fine, but it would seem the MBA doesn't recognize SDXC cards even though support for this was added after Snow Leopard.
Might someone have any suggestion as to what I can do or try to make it work with my MBA? I haven't had much luck with my googling.