That's something a lot of people get confused with. Don't worry: it's a really simple problem. You see, what you've imported let's call the Tileset material, but you still need to turn it into a usable tileset. To do this, go into the Database and click on Tilesets. Then, you need to add another space to the array. (So you have 51 tilesets, 1 of which is blank) Then, you go to the new, blank one and select your Tileset material and select which tiles are passable, not passable, obscured, countertops, etc. You can also select the panoramas and battle backgrounds here. Now you have a usable Tileset and can select it from the map properties window. Hope this helps!