Yes, I am a super noob. I'm sure the answer is very obvious but I haven't seen it yet.
I've downloaded a number of add-ons and while most of them work just fine, some of them are not appearing as options when I wrench a brick to generate new weapons. I've activated them in the "add-ons" pull up menu in the "Start a Game" screen, and while some items work, others just don't appear.
Also, in the add-ons forums, I can open a lot of the files, but the ones that appear as Xee files just open as images, not usable files. Again, I'm sure it's all wicked obvious to all of you much smarter, more experienced players, but please have mercy on a dumb noob...