The first time you run the pattern viewer and the first time you open a licensed pattern (like the ones from PatternsOnline.com) you need to be logged in as a user with administrator privileges.
This is usually the problem with that error message.
When I download the installer for the Pattern Viewer, all that is in the folder when I unzip is a "Floss" folder. So, I can't install because there is nothing there to install.
I have some patterns from patternsonline.com that I want to print and I don't have a windows machine anymore so it would be great if you could look into this.
I tried downloading again in case there was an error and had the same thing happen.
ETA: It turns out there was some problem with Stuffit so it was only extracting that one folder. I still can't figure out why it was doing that so I'm using the Unarchiver instead.