I suppose you are looking for the path to textures for free HQ Plants samples bundled with Forest Pack.
After successful installation they are by default located at:
C:\Program Files (x86)\Itoo Software\Forest Pack Pro\lib\Free Models\HQPlants Free\maps
Adding this path to your External Files list under Customize - Configure User Paths should fix the issue. For that purpose, there is a pop-up window when Forest Pack starts which can perform it automatically for you.