ok ive down loaded this ( USVIPER.zip here. ---> http://www.simviation.com/fs2004military2.htm) unziped to a temp. file, then copied and pasted to FSX\SimObjects\airplanes is their anything else that im missing to do......
Read my previous instructions carefully. Extract the zipfile to a temp folder & look for a folder inside it named VIPER. Paste the VIPER folder into FSX\SimObjects\airplanes.
If you do that it should be listed on the FSX selection menu.