"extra files" means no "folder in folder" and/or readme's and other files that should be not be in the Folder.
If you are open your CSO-Folder, only the "ovl"-Files (two) should be in and the Folder that includes the cso.
And i think it doesn't matter which system you use (mac or pc), the folder structure for both is the same, so the Screen from Cheleste should be an help.
Because it shows you, what files only be in your CSO-Folder.
I downloaded "Jcat's Pumper Steelworx" and tested it out myself, exactly the way that Cheleste wrotes in the other thread and it works fine in my RCT3.
Soo long, Cobra
btw, before you post -> look at the date from the last posts, i think Jinoc can't be answer your question