As the Width Selector tool has several keypresses for certain operations, we suggest installing the free Astute Graphics plugin Astute Buddy, which creates a panel that dynamically updates to inform you of the various keys which can be pressed in the tool’s current context.
The Width Selector tool works largely in conjunction with the associated Width Selector panel, which should be open and accessible. If you are using the free Astute Graphics plugin DirectPrefs, you can have the Width Selector panel automatically be shown when the Width Selector tool is selected.