[Seven Skins]

From your postbit template code in red color is missing add that in between the signature start and end hook:

        $template_hook[postbit_signature_start]
        $ad_location[ad_showthread_firstpost_sig]

        <if condition="$post['signature']">
        <!-- sig -->
            <div>
                __________________<br />
                $post[signature]
            </div>
        <!-- / sig -->
        </if>

        $template_hook[postbit_signature_end]