If you want the menu should be fixed even when you scroll down then you have to use css property postion:fixed and the demo site which you have provided did the same thing
<div id="top" class="wrapper">
<div class="wrapper">
<div class="logo left">
<img src="https://stackoverflow.com/questions/17133819/images/dipstrategy.png" alt="web agency"> </div>
<div class="navigation right">
<ul id="nav">
<li class="margin-right15 current"><a href="#home">HOME</a></li>
<li class="margin-right15"><a href="#do">WHAT WE DO</a></li>
<li class="margin-right15"><a href="#team">CORE TEAM</a></li>
<li class="margin-right15"><a href="#work">WORKS</a></li>
<li><a href="#contact">CONTACT US</a></li>
</ul>
</div>
<br clear="all">
</div>
</div>
CSS :-
#top {
position: fixed;
background: #1a1a1a;
width: 100%;
height: 70px;
z-index: 1999;
}
solved How to make header become static in the top [closed]