You can’t change the file extension from .rbz to .rb. That won’t work. The .rbz files are simply a .zip file with the extension changed to .rbz so that SketchUp knows that they contain a compressed set of .rb files. So what you can do is change the .rbz extension to .zip. Then you can double click on the .zip file and you will see the uncompressed contents. In CabWriter’s case it will be a folder named cabwriter and a file named cabwriter_loader.rb. Now you can simply drag and drop both the folder and the file into your SketchUp Plugins folder.
However, you shouldn’t have to do this. The Window/Preferences (SketchUp/Preferences on Mac) Extension, Install Extension button is the easiest way to install an extension with a .rbz file extension.