Ahh. That helps! I'm guessing the answer is to copy everything in C:/Users/Greg/Documents/ERP_WebbotLib to C:\Users\Greg\webbotlib.
I've not used the 'included directories' option. But its probably working as intended. I would guess that anything in \webbotlib can see stuff in the included directories, but not vice versa. (typically these included directories/libraries would be completely standalone, which is perhaps more common in non-robotics, e.g. including a library of ODE solvers)
The way I do my projects is I'll create a new folder, save the .prj file in it, and then generate the project in the same folder. Then I'll put all my own code in the same folder. Thus, all of my .c and .h files, and all of the webbotlib-created .h files (including hardware.h) are in the same folder as the .prj (or in some webbotlib-created sub-folders).
(More) What I do... Since Project Designer defaults to that C:\Users\Name\webbotlib folder, and I keep my project folders somewhere else, I have a shortcut in the webbotlib folder that goes to my actual folder full of projects.
Let us know if that solves things!