Well make sure the directory name is "p51d_1" through "p51D_15" and you should be good to go. If you don't have an object text file, it will show up as "p51d_1" in the drop box in the hangar. If it does have an object text file, it will list as the name of the plane, so open the "p51d.txt" file up and see what it's named, then look for that name in the hangar when offline.