Lute tablature: Adding bass notes below the staff

I am having trouble notating an 8th course in my lute tablature score. I want to put it on the 4th beat of the penultimate measure. I have specified an 8-course lute and added and defined the pitches in the String data dialog. I place an a on the 6th course (bottom line of the staff and use the Down key. First down - small blue box appears under the bottom line. Second push - a slanting line appears at the front of the box. I type a lowercase a. The last staff, where I tried to enter the note disappears, and a stack of ledger lines appears running from the next to last staff down off the page to the bottom of the window. I insert a photo.