If it shows up in-game, it will show up in the skin viewer, and vice versa.
Edit: Okay, let me clarify.... Do you have it showing up in game? If it's already in place, run the skin viewer, select your plane model from the drop box. The second drop box will have the names of directories (i.e. F6F5_1) and you choose the skin folder you are working in rather than the default.