Help please

Im new to this flight sim lark, and have bought FS04, Im doing ok and havnt crashed too often… 😮
This is probably a stupid question with a really easy answer but……..
Ive just gone onto avsim and downloaded a nice new shiny Saab Viggen to play with… but I cant find any information of how to add it onto FS. Ive done the obvious bits but without much luck
Ive also tried to play online but having troubles there too
Any help would do!