Search the official forums, ofp.info and ofpec addons depots to find the addon - they are the most likely sources. The readme may also provide a clue, or at least an email address for the author.
Phantom addons are sometimes a problem. Check the mission.sqm manually for references to it, not forgetting the intro and outro sections. Also check the scripts. If all else fails create a new map in the mission editor and copy and paste the mission into it, that sometimes does the trick.