Al,
It may be easier if you set your default text editor to 'Atto HTML editor':
Click on your name/icon in the upper right corner of the window. Select 'Profile', then just below your name/icon , click on the 'Action menu' (gear) icon, select 'Editor preferences', then 'Atto HTML editor'.
In this way, the Atto HTML editor will automatically be displayed in the quiz settings.