My computer does recognize that I've put an SD card in?

This is the first time I've ever used an SD card (because my new Nikon D3100 uses SD cards); before I used Memory Stick Pro Duo cards.
I know that I've put it in the right place though because the little green light thing is on, but nothing happens.
I thought that Autoplay would pop up onto my screen, but it didn't.
I also checked to see the devices plugged into my computer, and the SD card wasn't there.

Check to see if you have plugged in the SD card the wrong way and if not, under the SD card port there's small writing that tells you what the port supports. If you are trying to use an SD card that is not supported, it won't work.

Have you tried opening the drive letter for the card reader. My desktop has a few letters for each kind of memory card. Try opening Computer and clicking on each letter to see if you can find it.

In Windows Xp, try this: Control Panel>System>Hardware>Device Manager>Volume storages or something>. Select, right click and Enable.