First let me mention, that when installing our plug-in, please double check you are not running any 3ds max or related (like the VRay Spawner) service during the installation process. Sometimes, even closed, 3ds max might not shut down correctly and its processes are still running in the background. To check this you can go to the Task Manager by pressing Ctrl+Shift+Esc and make sure they've both have shut down correctly.
It looks like, that for some reason paths to Forest Pack's assets are missing / not properly mapped under your 3ds Max's configuration accessible by Customize - Configure User Paths - External files and stored in the 3dsmax.ini. The .ini file is located by default here:
%localappdata%\Autodesk\3dsmax\2019 - 64bit\enu\3dsmax.ini
Actually, it's not necessary to configure these paths manually. When Forest is loaded, it's checking if these paths exist and if not, all of them could be added automatically.
If you marked previously the Do not show this message again check-box to disable that pop-up window, you can revert it to its default value (to pop-up) by setting the checkTexturePaths = 1
in the forestpack.ini configuration file located by default at:%localAppData%\Autodesk\3dsMax\2019 - 64bit\ENU\en-US\plugcfg\forestpack.ini
Hope that helps and please let us know that's all working fine or you are still facing troubles with it.