From the left sidebar, click App Settings and select Menu from the dropdown menu.

Click Add Custom Button at the bottom of the page.

Add a Title and select which kind of button you'd like to create.

Select External Link and enter the URL of the page you'd like to link to.

To add a button for a Custom Page, select Custom Page, then choose which Page you'd like to link to.

Click Save Changes in that window, then scroll up to the top of the page and click the main Save Changes button. Your Custom Button has been added to the Menu!