Just adding the tileset into the folder isn't enough. Go to Database->Tilesets, and on the left side, click "Change Maximum" and add one to the value. Scroll down to the newest empty tileset and select the graphic for it. You'll also have to define Passage, Priorities, Bush Flags, Counter Flags, and Terrain Tags here (so the player doesn't go through anything or appear above a roof, etc).
Once you're done, press Okay and you'll be able to use it in a map.