Just forget the PCK file for now man.
Oi, I get the frustration but keep it nice. TwistedTwigleg is trying to help you out here and has given some perfectly good information.
To load in a scene and add it to the calling node you would do the following:
var some_scene = load("res://your_path/scene_name.tscn") # Load the actual scene 'blueprint'
var instance_from_scene = some_scene.instance() # Create a node from this blueprint
add_child(instance_from_scene) # Add the node to our active scene and tell it to process
It seems like you had most of this. If you are getting an error then you are doing something wrong. The only place I can see you going wrong, as TwistedTwigleg has implied, is that you are entering the wrong path to your scene.
You gave us this:
var next_scene = load("res://mods/" + modName).instance()
What is modName at this point? Double check that the variable is correct and that the path exists in your local project directory.
EDIT:
Just in case this is what you are trying, let me address it. If you are attempting to create a .tscn file separate from your compiled project and load it in as a mod then this isn't how you do it. You have to 'compile' the scene as a PCK (same way as you compile the whole project) and load it in that way.
res:// references the 'location' inside of a PCK file and not the actual file system that your game is launched from. Kind of, best way I can think of describing it.
