How to: Add, Edit, or Delete a Field in the Datasets Window (Report Designer)
In Layout view, select a dataset from the box at the top of the Datasets window.
To add a field, right-click anywhere in the list of fields, and then click Add. To edit a field, right-click an existing field, and then click Edit.
Type a name to use for the field.
Note: Names must be unique within the dataset.
Click Database field or Calculated field, and then type a value. For a database field, this must be the name of a field returned by the query in the dataset. For a calculated field, this must be an expression. Click Expression to build an expression.
Note: The expression for a calculated field cannot contain aggregates or references to report items.