Magento Shipping Extension自定义标签问题

I am developing a shipping extension. I would like to add a link under shipping label on onecheckout page. I tried putting some rewrite for block like:

<blocks>
    <modulename>
        <class>Company_Modulename_Block</class>
    </modulename>
    <checkout>
        <rewrite>
            <onepage_shipping_method_available>
               Company_Modulename_Block_Onepage_Shipping_Method_Available
            </onepage_shipping_method_available>
        </rewrite>
    </checkout>
</blocks>

I created the block as well and extends it with "Mage_Checkout_Block_Onepage_Shipping_Method_Available" and override getCarrierName($cc). But it is not working, Can anyone help, thanks in Advanced.