You have various options for entering line breaks:
With static text (no equals sign):
Edit the text directly in the Editing Pane, and press [Enter], or
edit the Value of the text field in the Expression Editor, and press [Ctrl]+[Enter].
With an expression (starting with an equals sign):
Edit the Value of the text field in the Expression Editor, and enter the Visual Basic constant vbNewLine. For example, the expression can look like the following:
="Retention" + vbNewLine + "Time"