Hello! This is a quick question and probably could have been posted in the help threads, but I'm here now so I'll try and help.
The problem is that Pepakura Viewer doesn't actually allow you to scale files. To change the scale you need to download Pepakura Designer. You can get the trial version for free from their website, which is fine if all you need to do is scale and print, just keep in mind that alterations you make to the files can't be saved unless you upgrade to the paid version.