Hi Douglas,
ESRI.ArcGIS.Client.Extensibility is part of our ArcGIS Extensibility SDK for Silverlight. We are in the process of posting the download for the SDK shortly.
Thanks,
Katy
All,
FYI, the ArcGIS Extensibility SDK for Silverlight is now available.
Thanks for your patience,
Jian,
Which version of the ArcGIS Extensibility SDK for Silverlight did you try to install? There is a version for 64-bit and 32-bit operating systems.
As for your second question, the ArcGIS API for Silverlight is the API upon which the ArcGIS Viewer for Silverlight is built. The ArcGIS Extensibility SDK for Silverlight is specific to the ArcGIS Viewer for Silverlight and ArcGIS for SharePoint and provides an API specifically for writing add-ins that allows you to access the map and selected layer, show user interfaces (UI) in dialog boxes, and store and load configuration data. The ArcGIS Extensibility SDK for Silverlight (and ArcGIS API for Silverlight) are required for developing add-ins.
See Extending the ArcGIS Viewer for Silverlight for more info.
Hope that helps,
Katy