adding link to magento top menu using layout update

Add link to Magento Top Menu using Layout XML update
in your custom module's  layout.xml default handle add this

      <reference name="root">
            <reference name="top.links">
                <action method="addLink" translate="label title">
                    <label>About Us</label>
                    <url>about</url>  <!-- can use full url also -->
                    <title>About Us</title>
                    <prepare>true</prepare> <!-- set true if adding base url param -->
                    <urlParams helper="core/url/getHomeUrl"/>
                </action>
            </reference>
        </reference>