EDIT: Here we go, manual page 784: Chapter 33 Working With Notation
To open the Guitar Tablature tab, do one of the following:
m Choose File > Project Settings > Score in the main menu bar (or use the Open Score
Project Settings key command), then click Guitar Tab (if not already displayed).
m Click the Settings button in the Arrange Toolbar, choose Score from the menu, then
click Guitar Tab.
m Choose Layout > Guitar Tablature.
m Use the Settings: Guitar Tablature key command.
m Double-click on the TAB clef, at the beginning of any staff that uses tablature in the
score.
Twelve different tuning sets can be defined. Each of them corresponds to one line in
this tab. The regular guitar and bass tunings are already included as defaults (first line
and last five lines), as are some of the more common guitar tunings.