I had this problem, and it was solved for me by following the advice at http://www.virtualroadside.com/blog/index.php/2013/03/11/virtualkd-2-8-with-virtualbox-4-2/ which tells you how to build an updated and compatible version of the Interop.VirtualBox.dll. You’ll need MS Visual C# installed, but the Express versions are free and work just fine.