With that file structure you should add whatever headers the library has to its “EigenPlugin.h” then for the game module or another plugins you just *#include “EigenPlugin.h” *without folders.
Looking at that screenshot, “Eigen/” folder doesnt exist and “StdVector.h” also doesn’t exist in the Source folder.
Everything C++ must be within the Source folder for a plugin; if you have external libraries then they have to go into a Source/ThirdParty folder and the code to link them is different.