Adapting site for mobile use

Specifically when viewed on a mobile the main header navigation menu is changed to Menu+
Many of our users do not understand that they should click on the + to see the menu (also they may not know what menu means in this context). I wish to change the word Menu+ to something like ‘Click here to see options’ but cannot find where to do this.
We are using our own theme based on the Blank Slate Theme and Knowledgebase themes.

The page I need help with: [log in to see the link]