To determine which command it is, choose the command on the left and click the chevron > (Windows, the Add button) to move it to the right. Not very descriptive, is it? What is that? Seeing a command in this list can be confusing at times.
In the list labeled "Choose commands from", choose MathType Tab (click screenshot for a full-sized view use the browser's Back button to return here. On either platform, you should now see "Word Options" ("Word Preferences" on Mac), already opened to the "Quick Access Toolbar" settings. If you prefer, on Windows you can right-click anywhere on any tab of the ribbon, and choose the command Customize Quick Access Toolbar.