I am using SSRS 2008 R2 and Report Builder 3.0. I set up a shared data source via the browser (http://myserver/reports/) wherein the \"credentials are stored securely in the rep
To accomplish what you are attempting, it sounds like you will need to use windows integrated security.
According to MSDN stored credentials are not passed to client applications. They are for use only by the report server for things like scheduled reports.
http://msdn.microsoft.com/en-us/library/ms159736.aspx
I can tell you that the prompt "Enter Data Source Credentials" is looking for SQL authentication, not domain accounts.