Try to close your li
tags properly:
<div id="header-bar">
<div class="container">
<ul class="nav">
<li><a href="#">Menu Item 1</a></li>
<ul>
<li><a href="#">Menu Item 1, Submenu 1</a></li>
<ul>
<li><a href="#">Menu Item 1, Submenu 1, Sub-submenu 1</a></li>
</ul>
</ul>
</ul>
</div>
</div>
--EDIT--
Use this at your css:
.header-bar ul.nav > li {
display: inline-block;
*float:left
}
It should make the trick.