After downloading and expanding the Cookie Monster ZIP file, you will get one .stack file.

Either drag and drop the .stack file to the RW8 / RW Classic application icon to install the file, or select the .stack file and choose Open With > RapidWeaver 8.app / RapidWeaver.app.

You will find two stacks inside the Stacks Library, one for the core functionality, and one for the iframe embeds.