[DustyPorViva]

Ok, first off, tilesets in the editor(or any graphics, for that matter) will not work unless they are indexed. If you have photoshop, open the image up and go to image>mode>index and click ok. If you don't have photoshop, I don't know how to help you there. Saving it in MSPaint though, will not index the image and will instead save it as 24bit(which the Editor can not render).

Otherwise, you want to do something similar to what alex said... but I'll guide you through the steps. His won't be able to be used offline, in the editor, though.

addtiledef imagename.png,levelprefix,0;

Now imagename.png is, of course the image of the tileset.
levelprefix is the prefix of the levels you want the tileset applied to. For example, if you have a gmap full of levels named city_a01, city_a02, city_a03 and so on, you can simply do:

addtiledef citytileset.png,city_,0;

That will make the tileset work for every level that starts with city_. If you want it applied specifically to a level...

addtiledef citytileset.png,city_a01,0;

Will apply it only to the level city_a01, no where else.