From version 5.0.13 the source code of Plugin Registration Tool is not available inside the CRM 2011 SDK (the executable is inside bin folder)
Because Microsoft only hosts the latest version, the source code isn't available anymore.
Someone has a link to download the SDK 5.0.12?
I asked this question on StackOverflow because that source code is a good example for some messages available for the CRM 2011 platform.