You need to unpbo the mission file found in the Operation Flashpoint/missions folder using a decryptor e.g UnPBO
Then put the unPBO'd mission into Operation Flashpoint/Users/=Your Name=/missions
Then open it in the editor, add the vehicles etc.
Then Re PBO it using an encryptor e.g MakePBO and put it back in the original folder..
easy stuff
I dont have the links for UnPBO and MakePBO but they are on here somewhere. I would read some tutorials first too on using the editor and how to UnPBO etc etc.
Welcome to the slippery slope of mission editing!!