I was looking for an answer to a similar problem and ran across this post. If it was never solved, here is how I got mine working: https://community.bistudio.com/wiki/Take_On_Helicopters_-_Rearmed
with the -mod paramater (Steam version of TKOH, GOG version of ARMA:CO), it looks something like this:
"-mod=B:\GAMES\ARMA II - Combined Operations\arma2.exe; B:\GAMES\ARMA II - Combined Operations\arma2OA.exe; B:\GAMES\ARMA II - Combined Operations\expansion; B:\Steam\SteamApps\common\Take On Helicopters\Rearmed; Rearmed"
Hope that helps anyone else who runs into this problem.