Installation <should> be a really easy progress:
1. Download the game, it comes in a .rar
2. Extract the .rar, it's a standalone, so you can extract it basically anywhere on your pc. Doesn't have to be with the rest of your Freespace installation. Though it could be, if you want to.
3. Use the launcher in the now extracted wings of dawn folder, check your settings in it, and run the game