I think you have to push in a little harder to eject it. Is it a Belkin card reader?
Well, I misread somethings. sorry about that. Ill try to explain it =/
When you insert the card, it slides in and secures itself in the card slot. To eject it (make sure you click "eject" before removing it, if it does that) push in, and it should sort of "pop" right back out, enough to where you can grab the card and slide it out.
I don't own a MBP so im not sure, but i know that on my old laptop, when i slid in a network card it would sort of "click", and the same for when I ejected it.