Usually this error is not fixed reinstalling Forest. Do you get it when creating a new fresh scene or loading an existing file ?
If the scene is new and the error is displayed inmediately, then the license file (forestpack.dll) may be invalid. Try searching it in:
\Documents and Settings\(user name)\Local Settings\Application Data\Autodesk\3dsmax\9 - 32bit\enu\plugcfg for Max 9, 32 bit
\Documents and Settings\(user name)\Local Settings\Application Data\Autodesk\3dsmax\9 - 64bit\enu\plugcfg for Max 9, 64 bit
(be care that some of these folder may by hidden in Windows). Also a copy of the file is stored in the 'plugcfg' directory below the Max root installation.
When you locate the forestpack.dll file, please send it to us for checking.
But, if you get the error from an existing scene and after working on it for a while, then the Forest objects may be corrupt. This is a common problem when the object was created using a pirated Forest Pro license. To fix it:
1. Delete all Forest and Planes objects
2. Save the scene and restart Max
3. Recreate the objects using your licensed Forest Pro
If your scene have a lot of Forest objects, you can also send it to us (only Forest/Planes and related objects, as area splines, lights, cameras and so). We may fix the corrupt items using some internal use utilities.
BTW, Forest don't use any registry value. The installer only copies some files to the Max directories, as described in our FAQ:http://www.itoosoft.com/english/menu.php?id=forest_faq#17