Two ways: FTP the folder that was extracted from the ZIP file to CubeCart's /modules/gateways/ folder, or obtain the lightning bolt token and enter it in CubeCart's admin, Manage Extensions page.
It will automagically appear in the Extensions list.
Welcome Welshy! Glad to see you made it to the forums.
The MarketPlace isn't a CubeCart store (at least, I don't think it is), so there isn't much other CubeCart users can help with this.
I think contacting CubeCart HQ directly will be the quickest way to let them know of your difficulties.
https://www.cubecart.com/contact