In the PDF documentation here, it says you can press the zero (0) key to enter a rest of a specific duration, so if you've selected a half-note, and press the zero key, a half-note rest is entered.
To switch an existing note to a rest, press the s-key. The s-key switches a note to a rest or a rest to a note.
This video shows using the s-key.
(no longer working)