Depends on which Menu Provider you're using and the attributes. For most my current menus, I've been using the DNNNav provider with the RenderMode set to display it as an Unordered List. In this case, you can apply the css rule margin:auto to the main class of the menu and that should align all the menu items to the center.
So the menu control in the ASCX file looks like this:
<dnn:NAV runat="server" id="dnnNav" csscontrol="MainNav" IndicateChildren="false" ProviderName="DNNMenuNavigationProvider">
<CustomAttributes>
<dnn:CustomAttribute Name="RenderMode" Value="UnorderedList" />
</CustomAttributes>
</dnn:NAV>
And the CSS would look like this (keep in mind, you'll have to adjust the attributes for your project this is a direct copy of one of my projects:
.MainNav {
font-family: Arial, sans-serif;
font-size: 13px;
width:620px;
margin: auto;
padding: 0;
line-height: 25px;
}
.MainNav .root {
display:block;
float:left;
list-style:none;
}
.MainNav .root.first a {
}
.MainNav .mi a{
display:block;
color:#666;
font-size:14px;
text-decoration:none!important;
padding-left: 10px;
padding-right: 10px;
border-left: 1px solid #FFF;
border-right: 1px solid #FFF;
}
.MainNav .root a:hover{
color:#F57B20;
border-left: 1px solid #CCCCCC;
border-right: 1px solid #CCCCCC;
}
.MainNav .m {
z-index: 9999;
padding: 0px;
margin:0px;
}
.MainNav .m .mi {
background-color:#FFFFFF;
text-align: left;
}
.MainNav ul.m {
border: 1px solid #ccc;
padding-left:0px;
}
.MainNav .m .mi a {
border: none;
padding: 2px 15px 2px 15px;
color:#666;
}
.MainNav .m a:hover {
color:#F57B20;
}