How do you read a SD card when your seasoned MAC does not have a SD reader?

I have an older desktop that does not have a card reader. I have an external SanDisk reader (ImageMate 6 and 1) that I connected via USB but I cannot register or read the data. A little more background - I'm trying to download mapping data from my garmin to a garmin software application using a 4 GB SD card. Is there a specific card reader that's compatible with my machine?