Providing you're waiting long enough (can take maybe 30 seconds on my old iBook), you're doing nothing wrong. The card should show up as a drive on your desktop, and, if your machine is set up that way, iPhoto should launch wanting to suck photos off the card. If the card is not mounting to your desktop at all, you should check that the card does work and does contain data (put it back in your camera, I guess), and check that your USB port works. Also check that the card reader works by testing with another computer, if possible.