LoL thats the loolest of the lolest answers I have heared. You have to change the size of the the doodad. Make sure that it is a doodad like a tree. Doodas like in the enviroment havent a size where you cant go into.
Doodads use things called pathing textures to stop units from walking through them (pathing maps for models) these are all a set size and cannot be changed thus you must use something else to stop usits from walking into them and pathing blockers are just the simplest.