No. Stephen King did not appear in the movie "Salem's Lot".
King's Salem's Lot tells the story of a novelist and a young horror fan who attempt to save a small New England town which has been invaded by vampires.
The movie starred David Soul, James Mason and Lance Kerwin. See full cast and crew at IMDb.
The author has appeared in George A Romero's 1981 film Knightriders and in a number of adaptations of his own books, including Pet Sematary, The Stand and The Langoliers.
See King's Movie/TV Appearances at StephenKing.com.