i had that tropuble so do this
Open it and next to planetype click "Browse" then select "stdshape", close the skin viewer and it should work
we u reopen it it should work and let u see the default skins,it wont let u see the custom skins,like the ones on ah skin site unless u down load them in to a folder in your ah skins folder eg a p51d u would make a new folder p51d1 and put the bitmap from the site in to the folder to view
hope this helps