Well...
First you usually want to scout the mission itself inside the editor. Just find any addons you accidentally placed there and delete them, until you think you might have gotten them all. Then you open the mission.sqm and delete the addons from the "requiredAddons" field, and you should be set.
However, sometimes there's just that one bastard addon that's hiding SOMEWHERE on the map, you just don't know where. In this case, all you can hope for is that the addon-maker has followed good addon-making praxis and applied his or her OFPEC tag to his addon. Simply find it out, and then in the mission.sqm, use Ctrl-F (search) for that tag, and it should show you all instances of the tag in the text.
And then you can just hope that the tag isn't something like "SOLD"
(be careful when deleting units like this, though: one }; too much and you might be crashin' your whole mission! Backup that mission.sqm!)
Good luck!
Wolfrug out.