Using font awesome in codepen
simply add this http://static.fontawesome.com/css/fontawesome-app.css in css setting panel:
It's working, you just are using the wrong font-awesome class.
Line 19 should be <span class="fa fa-search"></span>
.
See this updated CodePen
Note: You can also add CDNs in the CodePen settings so you don't have to include it inline in your html.