Search form with Combobox

Hi there… could any one help me?

i am trying to create a search/ filter form … but i am not able to do that … and i have been trying to do that for a long time… ??

The result is getting from a combobox (populated by a especific custom field) in the side bar, after selecting giving the posts with that custom field…

i have done this already

<form role="search" method="get" id="searchform" action="<?php get_bloginfo('URL'); ?> ">
                                            <fieldset>

                                                <input type="text" name="s" id="search" value="<?php the_search_query('meta_key=referencia'); ?>" />

<?php
                                        $cbox = '<select name="Concelho" style="width:291px; height:25px; text-align:left; font-size:9pt; padding-top:3px; margin-top:10px;">';
                                        foreach($Concelho as $key=>$val) :
                                        $cbox .='<option value="'.$val.'" selected="false">'.$val.'</option>';
                                        endforeach;

                                        $cbox .='<option value="" selected="true" >'.Seleccione.o.Concelho.'</option>';
                                        $cbox .= '</select>';
                                        echo $cbox;
                                        ?>

<input type="image" alt="Search" src="/lowcost/wp-content/themes/low-cost/images/Single Page/enviar.png" />

?? Any help?

Thanks in advance