Have you checked to see if the SD card reader is listed in system profiler? (apple menu, about this mac, more info button) it should be listed under Hardware, under USB, it would be labeled some thing like "Internal Memory Card Reader".
If the card reader isn't listed:
Have you tried a NVRAM Reset?
Have you tried a SMC Reset?
If the card reader is listed, but your not seeing the inserted cards:
Have you checked to see if the cards are reconised in Disk Utility? (go to the dock, click on finder, go to the menu bar, click on the go menu, chose Utilties, double click on disk utility) Do you see the sd card listed?
sticker side facing forward?
Software up to date? (apple menu, software update)
could try the NVRAM reset?
How many cards have you tested with? Dose any thing else read the cards?
could try a safeboot?
could try creating a new user account and see if that account has the same issue.