Hey recoculous,
thanks for having an interest in my work. First, this plugin isn't updated anymore. I hope most of the bugs are fixed and it works for most use cases.
Regarding your questions. You can double click a parameter to change it and it should show a description below.
You can also take a look into the plugin file itself. Search for END OF HELP and scroll down. This is where all the parameter definitions start. Each parameter has a line starting with @desc for the description.
The "Main Cost Items" are items you can define for a tree which will then be shown in the currency window (top right). Let's say you make a fire mage who unlocks some skills using an item called firestone. You can add the ID of firestone to "Main Cost Items" to show the number of firestones the player currently has.
To fit the whole tree on one screen you can either adjust the Node Visual Settings to make the nodes smaller or use a higher resolution (this can be achieved using Yanflys Core Engine Plugin for example)
Best regards,
Felski