You'll need to "unzip" the file you've downloaded! If your computer doesn't have a program that can do that already, you can try downloading WinRAR or 7ZIP, as a couple of examples that work for Windows. One you have one of those, just open the clangen file with that- it should ask you if you want to "extract" the files; say yes and have them placed anywhere you like. I recommend giving it its own folder to avoid misplacing anything! Once you have that done, just find something called "Clangen.exe" and run that :)