I cut and pasted the scripts from your article, and it is: ="Data Source=" & Parameters!ServerName.Value & ";Initial Catalog=master"

One thing I notice from my data source designer screen is mine had Edit button highlighted and the F(x) button greyed out. Yours is the opposite - F(x) highlighted and the Edit button greyed out. Is this something we set on server configuration?

I'm not able to insert screen prints from my designer screen here for you to see.