Thanks for checking out this mod and also for using Plus Porter, even after all these years. What steps did you take to apply the port to your CS+ installation? This is what you should do:
To use Plus Porter, you first want to go to File -> Open CS+ Directory, and browse to your Cave Story+ installation. To apply a port that has already been made, go to "File" -> "Open port...", browse to the folder of the port you wish to open, and click "Open." After the port has been loaded, go to "File" -> "Apply port", and a window will open up listing all mods currently installed in that game of CS+, and the order in which they'll be listed in the challenge menu (with the one at the top appearing on the main menu). You are able to arrange the mods in whatever order you want the challenge menu to list them, and then click the "Apply port!" button to apply the port to your Cave Story+ installation. Exit out of plus porter, and you'll now be able to experience the mod in Cave Story+.
Much to my shame, I will have to admit that I lost a bit of steam right near the end of Plus Porter's development cycle. During one of the last releases, I unfortunately broke some stuff (but I don't think anything critical), and I didn't realize it for a couple of years, and by the time I found out, I didn't have much motivation to fix it and haven't been good about setting aside the time for it. I also never made a video tutorial like many people were asking for, so sorry for any trouble you've been experiencing.