To use an expression instead of creating an ordinary selection/filter you do the following:

1. Drag out the relevant dimension from the list of dimensions as usual (for example, Account).
2. In the Layout Editor, click the X button to delete the filter that was created when you dragged in the dimension.
3. Open Expressions from the Report Designer menu and drag the relevant expression into the filter area.

NOTE: You can edit the expression from the Layout Editor; click the Edit expression button to go to the Expressions Editor.