To utilize use counts, click the small icon in the upper right hand corner of the Library pane (it looks like three horizontal lines with a downward-pointing arrow). A menu will expand; at the bottom of the menu you'll see "Update Use Counts Now" and "Keep Use Counts Updated." The first will check the current usage of all symbols against the current use count, and update the use count accordingly. The second will make sure that use counts are updated regularly.

