In the Score Editor it is easiest to enter and edit a track that you want to display with a drum clef by switching to a bass clef. To switch clefs, choose Track from the Display menu and change the Clef field. When you have finished entering or editing notes, switch the clef back to a drum clef. This is necessary because you can map several pitches to the same position on the staff when you display notes using a drum clef, making it impossible to edit properly. (Using a single-staff drum clef maps all notes to the same staff line.)
You can also enter or edit drum tracks using the Piano Roll Editor.