S should work in the Selection Tool. If it doesn't, create the metatool (Shift-S and select the style "Slash Notation" in the list). To apply, select the region, hold S and click.

Although Michel H is correct about the shortcut (S), you must be in the Staff tool.

Staff tool > click the measure(s) > S

This also works to convert notes to Rhythmic Notation with the shortcut R.

Oops, of course. Thank you, miker.