Hi Theo,
It seems that for some reason, paths to Forest Pack's assets are missing or not properly mapped in your 3ds Max configuration. By setting checkTexturePaths = 1 in the forestpack.ini, you shouldn't need to manually configure the corresponding path. When Forest Pack is loaded, it checks if the paths exist, and if not, it can add them automatically (a warning message will be displayed).
If you can't see such a window when starting the 3ds max for some reason, you can map it "manually" - you can access the configuration by going to Customize - Configure User Paths - External files and the relevant information is stored in the 3dsmax.ini file (C:\Users\<yourUsername>\AppData\Local\Autodesk\3dsmax\<maxversion>\ENU\3dsmax.ini).
By default, these distribution maps are located under the following folder:
C:\Program Files\Itoo Software\Forest Pack Pro\distmaps\images
I hope that helps.
Best regards,