Originally Posted by
Toyoka
There is a mini-menu in the editor labeled "Functions" which should have the 3 different functions (Terrain, Texturing, and Shading). Toggle the "Texturing" one, and also make sure you select a layer in the window that pops up when you hit the "Texturing" tab. I assume you have a texture already selected (if not, go to "Textures" and find one suitable for you), now hold the left mouse button while having the brush on the terrain, it should paint your texture onto it. Depending on what layer you selected, and whether there are other textures on the .ADT, you may not see your texture appear, or you may over-ride an existing texture.
P.S. Thanks, Reflection, I appreciate it! I would rep you back if I could but I gotta spread :\