The MiddleVR plugin is stored as a UnityPackage. To import the MiddleVR package, open the Asset menu, then Import package and Custom package :
You will find the MiddleVR UnityPackage in the data folder of
your MIddleVR installation, typically
(x86)\MiddleVR\data if you're on a 64 bits system) :
This will open a new Unity window :
The MiddleVR package contains libraries (DLLs) and Unity scripts that will manage all VR aspects.
Simply click "Import".
The package will then be imported and is now available in your project