Why won t my Mac register my sony memory stick pro duo magicgate mark 2 4gb memory card connect with my mac?

It worked fine in my old laptop which was an acer. I stick it in the thing in the side of my laptop meant for memory cards but nothing happens. I know the thing works because my friend and I tried with her memory card and it worked. My memory card is smaller than hers so do I need a card reader for it? It worked fine in my old laptop without one so I'm not sure.....