Click on the clef in the object type palette to select it and click on it again to display the clef palette. Choose the clef you want by clicking on it.
Make sure the pencil tool is selected in the toolbar. Now enter the clef you have selected at the point in your track you want it. Click at this point with the pencil tool and the clef will go in. Now all notes after the clef will be drawn to reflect the clef.
The five-line and single-line drum clefs should only be entered at the beginning of your piece. When you enter the single-line drum clef, the staff changes to a single line. All notes entered will appear on this line.