Did you install the text correctly? Then just go to Dialogs > Fonts. On the bottom of the window (all the way on the bottom right if I remember correctly) it'll say "refresh list" or something. Just click that and wait until everything's refreshed and most likely you'll have what you want =]
You press the text button in the gimp menu, then click where you want to put the text, then you type what you want it to say. If you mean that it isn't working though, I dunno how to help you.
Try in the GIMP menu go to the A click where you want to put the text, and in the box that pops up, type the text there, and go to dialogs>layers, and where all the layers are, you see an eye next to them, if the eye isn't there for the text layer, click where the eye should be, if not that try putting it in a different color
