Hi, Forest tries to load the ForestVRay??.dll files (there are several of them, for each V-Ray version). When one of then matches with the current V-Ray version, it's used.
These DLLs are installed in the 3DS Max root folder.
So, for render nodes, you must copy ForestPackPro.dlo file in the "plugins" folder, and all ForestVRay??.dll in the 3DS Max root folder.
For reference, the full list of files is describe here
Please note that if Windows cannot find the V-Ray DLLs in the Max root folder, it searches them in the directories defined by the PATH variable. (this is a standard behaviour for the DLL loading)
I guess in this case Windows is loading the wrong DLLs from other folder defined in PATH, because they doesn't exist in the 3DS Max root.