#1 First, make sure FSX is working as it should be. If not, clean install, and I think there is at least 1 update to FSX.
#2 Also, make sure your graphic cards drivers are up to date. I needed to get mine up to date for FSX to work properly with all the repaints I added.
#3 Okay, using the a26_soh.zip for manually inserting the 'Vader into FSX (not the a26soh.zip autoloader):
#4 Unzip the a26_soh.zip file into a folder for the Invader. WinRAR, 7-Zip, whatever you use to Zip and UnZip files. It will give you 3 folders (probably under an unzipped folder of a26_soh): Aircraft, Effects, Sound, plus the "follow directions" ReadMe.txt file.
#5 Look under the Aircraft Folder. Copy the folder, a26_soh (NOT the a26soh for auto install) from in the Aircraft Folder into: C:\Program Files\....(MS Games or whatever you have here)....\Microsoft Flight Simulator X\SimObjects\Airplanes. The A26_Checklist.txt file can be found under here as well.
#6 Copy all Three Files (fx cranky, fx twin tiremark, fx_vclight_day...) from in the Effects Folder into the C:\Program Files\.........\Microsoft Flight Simulator X\Effects folder.
#7 Copy the energizer file in the Sound folder into the C:\Program Files\........\Microsoft Flight Simulator X\Sound folder
#8 Restart FSX. Should work.
You may not see a little graphic on the plane selection screen, unless you move the thumbnail graphic to where it needs to be, but I don't bother. You should still see a description for the "Douglas A-26 Invader" in plane selection even if you don't get the thumbnail.
Don't worry about the repaints until you get the plane itself to work. Many repaints require you to play with aircraft.config files, and if you are not comfortable with that, you should not do it.
