There isn't one.
The MvvmCross Messenger is a 'pure portable' plugin - it doesn't contain any platform specific extensions, just portable code.
The nuget installers take this into account automatically - as shown in the n=9 video and source code - http://mvvmcross.wordpress.com
You can read more about plugins - including about how to write and load them in https://github.com/MvvmCross/MvvmCross/wiki/MvvmCross-plugins