OK, I'm not sure, but I think this problem has started since I've been using BetaPatch 1.96.
When I start any mission I've recently downloaded (I think since BetaPatch1.96) I get an error "cannot load mission, missing addon...". The mission still loads and plays OK (because I do have the needed addon). I did some playing around with some of these recent missions and found that when I "UnPBO" them the addons are not listed under the addons header in the Mission.sqm file. If, however I put the "unPBOed" file into my edit folder, then open it from the editor and resave it, the required addons are then added to the Mission.sqm!. I can then "rePBO" the file and it plays without the irritating "missing addon..." message. Has anyone else noticed this problem? Is it a BetaPatch1.96 issue? Any help would be appreciated because I do not like to have to run the editor on every mission I download that uses addons.