What card reader, what card, what MacBook and what Mac OS X version are you using?
Have you tried different USB ports on the MacBook?
Also did you have a look at Applications > Utilities > Disk Utility (DU) and saw if the SD card is shown in DU?
Can you see what file system (FAT32, NTFS) the SD card is using, while it is mounted on the Windows PC (select the SD card in Explorer and press ALT+ENTER to see informations about the card)?