I see from your signature that you are using 3ds max 2017 for which Mental Ray was an optional download. Could it be that you installed 3DS Max without Mental Ray ?
If that's the case then I would suggest you reinstall 3DS Max 2017 with Mental Ray included as it's currently required by our plug-in, even if you use other renderers.
I'm not sure if we'll be able to fix it in the short term, because Forest Pack has many dependencies to the mental ray libraries, but we´ll check what can be done to avoid similar troubles.
I hope that helps, please let me know if you have any further questions.