Unable to use Entityframework 6.1 with Prism 4.1

I've a Prism 4.1 Silverlight 5 web Project I want to use EntityFramework 6 with WCF RIA. Its not allowing me to add EF 6.1 from Nuget. Why? and what is the work around ?

http://stackoverflow.com/questions/24517333/unable-to-use-entityframework-6-1-with-prism-4-1