When i look in your source code i see this:
<style>
#mobile-header {display: none; background-color: #4c4c4c;}
.mobile_nav {display: none;}
@media only screen and (max-width: 585px) {
#mobile-header {display: block;}
#navDiv {display: none;}
/*.mobile_nav {display: block;}*/
}
</style>
Your mobile_nav wont go to display block because you forgot to remove the comment tags ( /* */ ).
Does this help??