You only browse for the stdshape.res file. This is in the root Aces High directory. You have to create your own skins folder and put the proper files in it. Once there you simply scroll down the list of planes in the drop-box at the upper right, choose your plane, then select the name of the folder with the skin in it from the secondary drop box immediately below this.
See the link in my sig for a webpage with a guide on how to create skins. If you don't care about creating new ones and just want to know how to preview, skip to the appropriate section.
Note: You cannot view existing skins in the skins viewer, only in-game. The skin viewer is for working on custom skins, rather than admiring the already-accepted ones.