As per @Dismissile suggestion upgrade your data access layer to EF6.
In EF6 the DbExtensions class still exists but has been renamed (see https://entityframework.codeplex.com/wikipage?title=Design%20Meeting%20Notes%20-(March%2021%2C%202013) to QueryableExtensions (see docs http://msdn.microsoft.com/en-us/library/system.data.entity.queryableextensions(v=vs.113).aspx).