mdtool Did not find registered GTK# installation

Question

I'm new to Xamarin Studio and mdtool so apologies for the noob question but I'm completely stuck on this and my Google searches aren't finding the answer to my problem.

Basically our solution builds perfectly in Xamarin Studio and now I need to run it from the command line so I can put it into our standard buildsystem but I appear to have missed something in the setup or configuration as it complains about the fact that it cannot find a registered GTK# installation.

I have installed Mono for windows 2.10.9 which contains GTK# 2.12.11 but I have also installed the standalone GTK# 2.12.20 because our developer says he has that installed and it is all installed on Windows Server 2012 in a VM at the moment while I test it all.

Here is the command I'm running to get it built and the eror that it responds with:

mdtool build '--configuration:Release' test_mono.sln

ERROR [2013-05-24 15:56:25Z]: Did not find registered GTK# installation
MonoDevelop Build Tool
System.TypeInitializationException: The type initializer for 'Mono.Unix.Native.Stdlib' threw an exception. ---> System.BadImageFormatException: An attempt was made to load a program with an incorrect
format. (Exception from HRESULT: 0x8007000B)
   at Mono.Unix.Native.Stdlib.GetDefaultSignal()
   at Mono.Unix.Native.Stdlib..cctor()
   --- End of inner exception stack trace ---
   at Mono.Unix.Native.Stdlib.free(IntPtr ptr)
   at Mono.Unix.Catalog.MarshalStrings(String s1, IntPtr& p1, String s2, IntPtr& p2, String s3, IntPtr& p3)
   at Mono.Unix.Catalog.Init(String package, String localedir)
   at MonoDevelop.Core.GettextCatalog..cctor()
The type initializer for 'Mono.Unix.Native.Stdlib' threw an exception.

Any ideas what I've done wrong or overlooked?