The solution suggested by designsimply (in the thread timethief linked to) is correct, of course, but it may not work if you simply copypaste it into the editor will at the same time retaining all the original stuff. For instance, at the moment you've got both of these in your CSS:

#branding .only-search + #access div {
    padding-right: 205px;
}
#branding .only-search + #access div {
    padding-right: 0;
}

So you should start by reverting to the Add-on mode, and enter only the changes you make.
For example, you had changed only the font size of #access ul. Under the Add-on mode, you'll add only this:

#access ul {
	font-size: 20px;
}

And when designsimply tells you that one of the changes you need in order to center the menu is this:

#access ul {
	display: inline-block;
	margin: 0;
	padding: 0;
}

you won't paste the whole thing, you'll add the rules under the selector you have already added:

#access ul {
	font-size: 20px;
	display: inline-block;
	margin: 0;
	padding: 0;
}

#access {
background: #c9b89b;
text-align: center;
clear: both;
display: block;
float: left;
margin: 0 auto 6px;
width: 100%;
}

#access ul {
font-size: 20px;
list-style: none;
margin: 0 0 0 -.8125em;
padding-left: 0;
}

#access li {
float: left;
position: relative;
}

#access a {
color: #000000;
display: block;
line-height: 3.333em;
padding: 0 1.2125em;
text-decoration: none;
}

#access ul ul {
display: none;
float: left;
margin: 0;
position: absolute;
top: 3.333em;
left: 0;
width: 188px;
z-index: 99999;
}

#access ul ul ul {
left: 100%;
top: 0;
}

#access ul ul a {
background: #ffffff;
border-bottom: 1px dotted #000000;
color: #000000;
font-size: 18px;
font-weight: normal;
height: auto;
line-height: 1.8em;
padding: 10px;
width: 168px;
}

#access li:hover > a,
#access ul ul :hover > a,
#access a:focus {
background: #ffffff;
}

#access li:hover > a,
#access a:focus {
background: #ffffff;
/* Older webkit syntax */
color: #000000;
}

#access ul li:hover > ul {
display: block;
}

#access .current-menu-item > a,
#access .current-menu-ancestor > a,
#access .current_page_item > a,
#access .current_page_ancestor > a {
font-weight: normal;
}

See http://en.forums.wordpress.com/topic/twenty-eleven-aligning-to-center-the-navigation-menu-css?replies=6