WEU questions do not belong here.
Anyway, to use more tilesets than WE normally allows, open up the WEU enhancer and click on create map. Then down at the bottom of that window it lists the tiles currently in the map, (Ashenvale is the default) then just above that it says Use custom tileset, check that box and then click on the button next to that, that says ADD, click on that then just click on the pic of the tile you want to add, voila, you're done. Now just click "Create map".
~Deathcom3s