I don't have Mepis in front of me (I'm currently in Ubuntu), but I believe GAIM should be there. If not, you can always add it in:
1. Go to Synaptic Package Manager (should definitely be in the KMenu somewhere)
2. Click "Settings," then "Repositories." You'll see a bunch of boxes that are checked and unchecked. Make sure they're all checked except the non-us ones. Then click OK or close or whatever gets you out of that window.
3. Click "Reload."
4. Then, once it's reloaded, click "Search." Type in gaim and Enter.
5. Now, if GAIM comes up and there's a green box next to it, it's already installed on your computer. For some reason, there's just no icon for it. If it's an empty box, go ahead and double-click it. Then, click "Apply."
You should be all set.