There's a lot of articles on the web about installing RAM (there's also tons of youtube videos). Here's a couple:
http://www.ehow.com/how_895_install-ram.html
http://www.build-your-own-computer-t...stall-ram.html
I know you're not installing, but there are pictures (click on them to see in high resolution) that shows you where to look, and what you want to reseat.
The important things to remember is: power HAS to be off, and you have to protect yourself from static electricity (ground yourself - touch the back of the computer case before you touch anything inside).
If you have any doubt as to how much RAM you have, when you start your computer, it should show how much RAM there is (during the POST - power-on-self-test it should run a memory test - saying nnnnnn KB OK).
If you're still low on RAM, and I think this may be what the guy did, you can try to increase the size of your page file (virtual memory). The page file is a file named pagefile.sys that uses hard disk space as if it was RAM (that's the notion of virtual memory). Of course this is slower, because hard disk access is much slower than RAM, however it allows you to use more space if you need it. To increase the pagefile size, right-click my computer, click properties, then go to the advanced tab. (I'm on XP at the moment so I don't recall exactly where it was). Then it should be under settings for performance. Then click advanced and you should get to the settings. Pick a maximum size and a minimum size, then
click SET. If you don't click set it won't change anything (very typical mistake). You may or may not have to reboot afterwards.