WOAH! no need to re-install XDD
I had this probelm before, any items you download from others MUST be put in a custom group, for instance, if you download a tree mesh, DO NOT put it in MyTrees, otherwise it won't show up in game (it will show in map maker, but not IG), make another group (EX. Cutsom Trees [as a group name]) for your downloaded meshes, and this will fix your problem. ;3