Hi there! Welcome to the forums!
Firstly, there's a tutorial for installing addons using the mod folder method, which almost everyone here will agree is better than installing addons to your root Addons folder.
You can find it here:
http://www.armedassault.info/index.php?gam...s=faq&id=34The issue could be down to required addons/patches that aren't installed, or an incorrect installations.
If you could let us know which addons you're trying to install, and exactly what files you're putting into the addon folder it should be easy to isolate the problem.
If you could also let us know what version you're running, that'll also help a lot.