When creating a park/roller coaster for the Build it Competition, can you use structures created by other users?

as long as you state that they arent your structures, i dont see why not. i never saw anythig in the rules saying everything has to be your content.

If you can use custom scenery from other users, you should be able to use custom structures from other users as well since it is essentially custom scenery in a different form. Like Super Moose said, give credit where credit is due and you should have no problem.