Mary, you should have 3 files in the extracted folder... Readme, Setup and setup.gcd. You don't open up the setup.gcd file. Ignore it. The only file you click on to install is the plain Setup file with the "key" icon. I'm not sure what it all means, but the .gcd file is part of their "wrapper" and is necessary for correct installation. If you read the readme file, it explains it all.
Heading out to the gardens!