Wilbur is IMHO not the tool to use.
DL a small file from the editors depot called image2ascii.exe
Then in any paint program draw a greyscale bitmap 256 x 256.
Black is lowest texture, white is top.
Use the in between colours to make your terrain.
drage the finished image and drop it on image2ascii icon. Then make lowest -5 and highest 200.
Then start wrpedit and import 1 1 . (normally with wilbur its 3 3 .)
I find I have much more control over the terrain with this method.
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Wilbur
There is an icon on the left that looks like a styized 'I' click on this and set the Tool Intensity to 0.01. That should do the trick.