I do all the time. If you installed FO3 outside Program Files and installed FOMM into FO3's folder, you have enough privilege as yourself to do anything FOMM can do.
Well I've done all that but when I hit 'open' it says to go to the readme.
Well, I got my first mod to work! Thanks to everyone here who helped me out. I don't want to seem like I'm just choosing people, but I don't think I would have got this done without Arwen and Dogsbody.. Yet, there's still a small issue. In the documents, files shown as RAR are the only ones showing in FOMM, ones like ESM don't. That's not good because the better mods are ESM, like Fellout and the beret mod, and I want both of them. So, I still need to find out how to get those.