It should come up with a few file locations, the first of which will probably be /usr/bin/gaim. If you type that in the terminal, gaim will open - alternatively, just open the "Run Command" box and simply type "gaim", minus quotes.
Gaim should be included in the Mandrake installation cds.Try opening the Mandrake Control Center (sic)-click on the icon in your taskbar that looks like a TV set with a red spanner, enter your root password and click on Software Management. Go to the top right "RpmDrake helps you remove software packages", and then "All packages, alphabetical" Any Mandrake rpm you install will be added to this list, so if you don't see gaim there, you have'nt actually installed it.
If you go to the other option "RpmDrake helps you install software packages", choose alphabetical again and scroll down the list, you should see gaim there. Click in the tick box, click install, and you'll be asked for the relevant installation cd.
It *should* be as easy as that! If none of this works, then I'm pretty much out of ideas....