Hi there Nick, long time no see!
There is probably a very quick solution here, and it would explain why you are getting the message, as well as to why the standalone version of Guitar Rig is working fine.
Firstly, GTP4 has to know where the VST version of Guitar Rig is on your system. If you've reinstalled Guitar Rig to a folder that isn't included in GTP's list of plug-in folder, you will get the error message.
If it is in one of those folders, just run the VST scan/adapter from within GTP4 and it will then be available to use.
If it isn't in one of the designated folders, you can either copy the plug-in's
.dll file into one of the designated folders, or start the VST scanner and add the folder the .dll is in to the list. Either way, you will need to run a full VST scan afterwards.
If you're not certain where the .dll installed to, and you allowed it to install to the default location, just run the installation disc and check where that folder is created (before, obviously, ending the installation process).
Hope that gets it working for you.
Steve
post edited by SteveStrummerUK - 2013/01/08 12:16:51