There are two possibilities:
That would be the easiest to correct, simply restart your computer, and there is a possibility that your BIOS didn't saw the added RAM the first time. Those thing aren't good and can possibly make mistakes.
Try removing the RAM and putting it back... Try switching the position in the slots - just make sure both sticks run correctly with your computer and that your motherboard can deal with the new RAM you installed.
From here on, or you solved your problem or you still have the problem:
Case 1: When you switched slots, you started the computer to be welcomed with an error telling you have no more RAM
This mean that the RAM stick you brought doesn't fit your computer/motherboard or that the RAM is DOA. If you know for sure you have correct RAM, DoA (Death on Arival) seems to be your case, It happens to the best ;)
Easiest fix: try to get a refund (if DoA, it will be easy), find your motherboard manual and get correct RAM for it.
Case 2: When you switched RAM, you still had only 2Gig on your computer
That means that you probably have a problem with one of the RAM slots on you Computer... Or, you motherboard can't deal 4Gig of RAM (which is possible).
"Easy" fix: get a new motherboard? If you have more than 2 RAM slows, try them out... or deal with 2Gig?
If you have other results, comment, I'll do my best to answer. And before buying that new motherboard (if you get there), try going somewhere and getting a second opinion, it never killed anyone and can save you money and time, as finding a new motherboard for a already existing computer is a pain.