From the Edit menu, choose Selection, and then choose Mark Out to set the end of the loop region at the current cursor position.

You can then right-click bar at the top of the loop region to insert a region, toggle looped playback, or select the loop region.

Select the Update loop bar on Mark In/Out check box on the Editing tab of the Preferences dialog if you want the loop bar in a  data window to be updated when you mark the beginning or end of a selection. When the check box is cleared, the loop bar isn’t updated until after you’ve marked both ends of the selection.

Marking the End of a Selection