Hello Charles! Here are the steps Marc mentions above - I’ve had trouble with this too, if you don’t do these steps in the right order it can be finicky. But overall Dorico is absolutely fantastic - especially the ease of writing without time signatures, if you desire.
- Select music.
- Set tonality to 24 EDO.
- Type Shift-K (there’s no need to deselect or reselect anything before you do this).
- Type atonal.
- Hit return.