That is very strange. Is it possible that it's not inserting all the way into the USB slot? Have you tried a different slot, or a hub?
If you go into the Mac's System Information Report window, can you see the Reader under USB devices?
culbe wrote:Another thing I'm curious about is when I plug it back into my PC it thinks there is only 55MB of space on a 4GB card?
Not sure I'm following you. Earlier, you said it worked on your PC?
Can you try it with a different microSD card?