This is very big problem to center a nav menu of twitter bootstrap. I have created a complete centered aligned navbar so you can copy its code and paste where you want Bootstrap Navbar Center.

Just follow these simple steps
Css Style of center align nav bar

#main_menu{
    text-transform: uppercase;
    margin: 0px;
}
#main_menu.navbar-default .navbar-brand {
    color: rgba(119, 119, 119, 1);
}

#main_menu.navbar-default {
    font-size: 17px;
    background-color: rgba(0, 0, 0, 1);
    border-width: 0px;
    border-radius: 0px;
}

#main_menu.navbar-default .navbar-nav{
     float:none;
        margin: 0 auto;
        display: table;
        table-layout: fixed;
}

#main_menu.navbar-default .navbar-nav > li{
    color: red;
}

#main_menu.navbar-default .navbar-nav>li>a {
    font-stretch: expanded;
    font-size: 18px;
    font-family: "Lighter-Font";
    color: rgba(255, 255, 255, 1);
    background-color: rgba(248, 248, 248, 0);
    line-height: 35px;
    border-radius: 0px 0px 5px 5px;
}
#main_menu.navbar-default .navbar-nav>li>a:hover,
#main_menu.navbar-default .navbar-nav>li>a:focus {
    color: rgba(255, 255, 255, 1);
    background-color: rgba(94, 180, 255, 1);
}
#main_menu.navbar-default .navbar-nav>.active>a,
#main_menu.navbar-default .navbar-nav>.active>a:hover,
#main_menu.navbar-default .navbar-nav>.active>a:focus {
    color: rgba(255, 255, 255, 1);
    background-color: rgba(94, 180, 255, 1);
    border-radius: 0px 0px 5px 5px;
}
#main_menu.navbar-default .navbar-toggle {
    border-color: #5eb4ff;
}
#main_menu.navbar-default .navbar-toggle:hover,
#main_menu.navbar-default .navbar-toggle:focus {
    background-color: #5eb4ff;
}
#main_menu.navbar-default .navbar-toggle .icon-bar {
    background-color: #5eb4ff;
}
#main_menu.navbar-default .navbar-toggle:hover .icon-bar,
#main_menu.navbar-default .navbar-toggle:focus .icon-bar {
    background-color: #000000;
}

HTML for bootstrap center aligned menu
<div class="navbar navbar-default " id="main_menu" role="navigation"> <div class="container-fluid"> <div class="navbar-header"> <button class="navbar-toggle" data-target=".navbar-menubuilder" data-toggle="collapse" type="button"><span class="sr-only">Toggle navigation</span><span class="icon-bar"></span><span class="icon-bar"></span><span class="icon-bar"></span> </button> </div> <div class="collapse navbar-collapse navbar-menubuilder"> <ul class="nav navbar-nav"> <li class="active"><a href="https://www.blogger.com/">Home</a> </li> <li><a href="https://www.blogger.com/products">About</a> </li> <li><a href="https://www.blogger.com/about-us">Services</a> </li> <li><a href="https://www.blogger.com/contact">Products</a> </li> <li><a href="https://www.blogger.com/contact">Gallery</a> </li> <li><a href="https://www.blogger.com/contact">Testimonials</a> </li> <li><a href="https://www.blogger.com/contact">Shop</a> </li> <li><a href="https://www.blogger.com/contact">Faq</a> </li> <li><a href="https://www.blogger.com/contact">Find Us</a> </li> <li><a href="https://www.blogger.com/blogger.g?blogID=2930196222246939663#" id="open-contact-form">Contact</a> </li> </ul> </div> </div> </div>