Adding a "three line" character (☰) or image to TB Mega Menu dropdown button
element:before {content: "☰";} should do the trick. I'm seeing some other problem in there actually. The presence of &lt;i&gt;&lt;/i&gt; element means there should be a font icon on load, with the i:before reacting as your three bars. Sirate theme, for example, doesn't have it, so it has to be a bug.

Could you provide a live site so we could give you a solution based on it? TB Megamenu could be different as markup depending on the theme. Or your CSS could be changed, based on some Zen stuff.