Thank you for all your kind suggestions! I've discovered it was the packages themselves that were the problem. I'm not sure what was wrong with them-- the artist said they were for Vista/7, and they showed up the same as the sample logon I downloaded, but they still don't work. The sample one did, though, so I know there's nothing wrong with my computer or program. Again, thanks a lot for your help!
Cavan1: I only found out about LogonStudio through deviantArt, actually, browsing their Vista customization deviations, so I know they have some. If you're interested, the package I downloaded was here: http://stamga.deviantart.com/art/Fresh-Logon-Pack-163422111