Hi Ralph,
The easiest method:
1. Enter a single note (eg. whole note) for the note duration that you want the roll to extend for.
2. Select the note, then click on the "Ornaments" button in the Notes and Rests Palette. In that toolbar, click the "Single Chord Tremelo" button. It will give you the tremelo, albeit it will be a mechanical sounding one (reminiscent of machine gun fire, depending on the quality of the playback device. The notation will be much cleaner, though
Second method, for better performance:
1. Enter the notes for the tremelo and massage the performance as desired. I'd suggest doing this before entering other notes in the measure, just to keep things easier.
2. Now click-drag to select all the notes, and click on the same button as in step 2 above. The notation should "coalesce" into a single tremelo notation, while keeping the performance that you worked on in step 1 here.
Enjoy
Sherry