Social Networking

Very STOOOPID NOOB question about Facebook. OK. So my g/f and I have both joined Facebook. I've accepted her as a Fb friend. We're both new to Fb so we don't know anything much about it - how do we get so we can talk to eachother in the little chat windows or in Messenger (if it's not the same thing?) It says she's my friend on her page, but when I click on the top of the little chat window that raises up the little box you can type in, her name's not there so I can't talk to her.

Facebook's not very user friendly so I can't see anywhere to show me how to do this. I just want to be able to talk to her on Fb. Thanks in advance.

Yours hopefully

Chris. (Please respect the word NOOB up the top and keep the answers idiotproof, I'm NOT good at the Net at the best of times and this sure isn't one of them!)