This is a custom stage based on Ricco Harbor in Super Mario Sunshine. The .zip file inside the original one is a complete MEX package that can be imported using the mex-tool. The model needs to be cleaned up, and the map looks odd when viewed through the camera, but that is all. (Oh, and an icon. I need an icon.)
Download the zip. Extract the first zip (the second one will have a .png and a .zip file). Then, in mex-tool, go to Stages->Import and select the downloaded zip file. Add to the Stage Select Screen (SSS) manually.