The invisible target addon may not be hosted at OFPEC, not sure.
Adding trees is easy, you just need an Editor Addon. The best one is General Barron's version, which is available in the
pending submissions section of the Ed Depot. It is fully compatible with the more famous version by Kegetys and all allows you to place trees and anything else you like.
(If you have 1.46 use Gunslingers' version.)