Hi,
The plugin works for both navigation bars. If you use only one navigation bar this one will be replaced by the menu button, if you use both navigation bars in Suffusion, both of them are replaced with only ONE button and the new menu will combine both menus from both navigation bars.
This also solve the bug with 2 select menus displayed in Navigation Bar Above Header if you use Suffusion native select lists.
If you don’t see the button on your site, maybe a style from your site prevent it to be displayed. by eg. your navigation bar is larger than the wrapper in responsive mode and the button is moved too much on the right.
Can check if your site is settled right to be responsive at: https://www.drafie-design.nl/set-suffusion-to-be-responsive/
Also, the URL of your site may help me to isolate the problem and to find a solution.
Thank You,