Hi James,
I have actually reproduced the issue you mentioned regarding Prism 4.2 Pre-Released nuget package. This package would contain the following assemblies taking into account that it is a beta realease:

• Microsoft.Practices.Prism.dll
• Microsoft.Practices.Prism.Interactivity.dll
• Microsoft.Practices.Prism.SharedInterfaces.dll
  

You could verify the assemblies available in the Nuget package with the Nuget Package Explorer which you may download it from the following site:

• Nuget Package Explorer
  

Therefore, you would need to manually add the Prism.MVVM assembly into the corresponding project until it gets added to the Nuget Package.

Regards.
Gabriel Ostrowsky
https://blogs.southworks.net/gostrowsky