I was working on a report in Report Builder 2.0 for SQL Server 2008 Reporting Services, and added a textbox to an existing report.  I wanted to display some summary info related to the data shown in the report.  I added a textbox, then went to edit the “Expression” property of that report item, but wasn't able to select any fields because it said “Report item not linked to a dataset.”

So I tried to find a property on the textbox related to a "Dataset" (like the "DataSetName" property on Tablix elements), but the only thing I found was a “DataElementName” property … which I tried, but didn’t work.

I then learned you can link to a dataset by defining it’s name directly in the expression, like this:

=Count(Fields!Address.Value,"DatasetName")