There reason WEU, wont work is because that was made before patch 1.22 came out and hasent been updated since. So for new versions of the editor and such it doesnt quite work..so from what i can tell there are two options..The first and probably most easiest would be downloading the Jass NewGen Pack..Which can be found here under Xantans post of World Editor Enchantments. Then go under Grimoire and enable no limits. (Sorry, i don't actually have the program so i cant help ya much here but that should be it.)
If that doesnt work, which i dont see why, theres a way to bypass that, just a little unorthadox. Just create a new unit. Change the Model to whatever doodad you want, give the unit the ability Locust, take away the attack and everything else and just use that instead.
However, that has its limits. Since it makes new units, your unit count goes up..and i cant really find out a way for that, and its only really useful if your making a map where the trees or doodads wouldent be attacked and such..if your making a tree tag map or something that destroys them, than in no way should this be used.