Welcome!,

Did a little digging around both on here and the internet and found a couple of really good examples showing how to do exactly what you are asking about.

Kathi Kellenberger's sitehttp://www.bidn.com/blogs/KathiKellenberger/sql-server/3082/configure-ssrs-to-use-a-dynamic-connection-string

http://skamie.wordpress.com/2010/12/17/ssrs-changing-data-source-dynamically/