For builds after the Beta 2, the IVsBuildManagerAccessor included in the Beta 2 bits not longer works.
These changes are documented in the document "General Notes Including new IVsBuildManagerAccessor.docx", located in the Documentation folder of the Beta 2 zip file release.
We are currently working towards getting these changes applied to the codebase, and plan to make another release shortly.
Visual Studio Platform
Program Manager - MSBuild