The first time you use Report Builder 2.0 for SQL Server reporting services, you'll want to define your SSRS management site name. This allows Report Builder to communicate with the SSRS site to retrieve site settings, DSN names, and path names to publish reports.
To set this option - Click the option button (upper right):
Then choose Options:
Fill in the URL to your reporting server management site.
Not sure where that is located? Browse to the SQL Server> Configuration Tools> Reporting Services Configuration Manager
Locate the Web Service URL in the left margin. Use the http://<reportserver>/<Virtual Directory> name for the Report Builder dialog shown above.