Click on the 'A' icon to display the note names.
Click on the '1' icon to display the interval names.
Click on the 'empty marker' icon to hide all labels.
Click on the semitransparent 'A' icon to show/hide the labels on the empty frets.
Click on the 'ABC/DoReMi' icon to toggle between Italian and international notation.
You can chose to hide the labels on some layers, this gives you the possibility of showing note names or intervals only on important notes. Look at this example. (Open this diagram in the editor)