MEF export IVsReferenceProviderContext never loaded in VS 2012\2013

Hi.

I've attempted to write an extension package (provide my own  IReferenceProvider and an IVsReferenceProviderContext) in order to extend the project "Add Reference" UI as described here under "Adding a Custom tab": http://msdn.microsoft.com/en-us/library/hh873125.aspx

I've also specify my MEF Component in source.extension.vsixmanifest. But my IVsReferenceProviderContext object is never loaded :(

Is there an additional step/mechanism needed to register my IReferenceProvider\IVsReferenceProviderContext in order to get it picked up by ReferenceManger?

Thanks,

Vitalij.