Hi Steve,
Don't you worry, I'll get you unstuck in a jiffy.
The Arduino IDE is a bit primitive and doesn't give you any options to organise nicely a project structure so everything ends up as random collection of stuff thrown in a single folder (I didn't design Arduino IDE so can't blame me :-) if you were using PlatformIO you would have a different experience and the project compilation from the Code Builder would make a lot more sense.
All those files in the "src" folder are required, every plugin is a standalone c++ class (like a mini library) and the starting point of the program that you can edit is the .ino file
You should also have an instructions file inside the downloaded zip but the general idea is that what you download is a self contained entire project that is also portable.
Here are the instructions what you need to do:
1. Copy the folder that is
inside the zip to
Documents > Arduino and open the Arduino IDE (if it's open close it first)
When I say "Documents" that's the user's
My Document on Windows or
Documents on a Mac, inside you will find a folder Arduino and that's where Arduino IDE keeps all it's internal mess (libraries and project files)
That's it, when you re-open Arduino IDE the project will appear in the list of projects and you can edit the .ino as much as you like.
I am not entirely sure what you've meant by "the main .ino file is locked" so let me know if you still can't open the project
G