In WED select the object and tick Polygon or BBox in its Properties.
With Polygon ticked the collision will be calculated with the polygons of the mesh, with BBox ticked the collision will be calculated with its Bounding box.

trueSpace 7.6, A7 commercial
Free gamegraphics, freewaregames http://www.reinerstilesets.de
Die Community rund um Spiele-Toolkits http://www.clickzone.de