So two notes:
1) To answer your questions, it sounds like the project didn't get fully unzipped, and that some of the plugins are missing or in the incorrect folder. Make sure you extract everything and keep the file directory the same. Rather than PM_event_platplus_state_script, it should read "Attach a Script to a Platformer+ State." You can find the OnInit for the scene by clicking anywhere in the scene that isn't an actor or a trigger.
2) Some of the techniques for the bounce are a bit advanced, I'd recommend you play around with GBStudio a bit before trying to replicate that particular bit. The Discord is a great community for learning the ropes, and also a really good place to ask questions!