codice:
<form action="add.php?cerca=&idcampo=&idpren=&idcliente=&ordinamento=dt_data" method="post" enablecab="No" enctype="multipart/form-data" onSubmit="return validateForm();"> 
    <table width="100%" cellpadding="0" cellspacing="0"> 
        <tr align="center" bgcolor="fbb666"> 
            <td colspan="2">Inserimento nuova prenotazione</td> 
        </tr>
        <tr> 
            <td>Cliente</td> 
            <td>
                <select name="idclientesel"> 
                    <option value="1" >Comedini Luca</option> 
                    <option value="5" >Uva Anna Elisa</option> 
                </select> </td> 
        </tr> 
        <tr> 
            <td>Campo</td> 
            <td> 
                <select name="idcamposel" onChange="submit();"> 
                    <option value="1" >campo da calcetto - campo esterno da calcetto</option> 
                </select> 
            </td> 
        </tr> 
        <tr> 
            <td height="2">Data prenotazione</td> 
            <td height="2"> <input type="text" readonly="yes"     name="data" id="data" onChange="submit();" value="">
                [img]../calendario/img.gif[/img] 
                <script type="text/javascript"> 
                     Calendar.setup({ 
inputField : "data", // id of the input field 
ifFormat : "%d/%m/%Y", // format of the input field 
showsTime : false, // will display a time selector 
button : "bottone", // trigger for the calendar (button ID) 
singleClick : false, // double-click mode 
step : 1 // show all years in drop-down boxes (instead of every other year as default) 
}); 
                </script> 
            </td> 
        </tr> 
        <tr> 
            <td height="2">Orario di inzio</td> 
            <td height="2"> 
                <select name="id_orainizio" onChange="submit()"> 
                    <option value="">- Orario -</option> 
                </select> 
            </td> 
        </tr> 
        <tr> 
            <td height="2">Orario di fine</td> 
            <td height="2"> 
               <select name="id_orafine"> 
                    <option value="">- Orario -</option> 
               </select> 
            </td> 
        </tr> 
        <tr> 
            <td height="2">Confermato</td> 
            <td height="2"> 
                <select name="bl_confermato" id="bl_confermato"> 
                     <option value="1">Sì</option> 
                     <option value="0" selected>No</option> 
                </select> 
            </td> 
        </tr> 
        <tr> 
            <td>note</td> 
            <td> 
                <textarea name="ac_note" cols=50 rows=5 id="ac_descrizione"></textarea> 
            </td> 
        </tr> 
        <tr bgcolor="fbb666"> 
            <td colspan="2"> </td> 
        </tr> 
        <tr> 
            <td colspan="2"> </td> 
        </tr> 
        <tr> 
            <td colspan="2"> <input type="submit" name="invia" value="Aggiungi" class="button"> 
                <input type="button" value="Ritorna al men&ugrave" onClick="location.href='index.php?cerca=&idcampo=&idpren=&idcliente=&ordinamento=dt_data'" class="button"> 
            </td> 
        </tr> 
        <tr> 
            <td colspan="2"> </td> 
        </tr> 
    </table>
</form> 

<script language="JavaScript"> 
function validateForm()
{ 
    errore='ATTENZIONE!! i seguenti campi non sono stati compilati:\n'; 
    if(isNull('data')) 
    {
        errore+='. scegliere la data'; ok=false
    }; 
    if(isRangeSel('id_orainizio','id_orafine
')) 
    {
        errore+='. l\'orario della prenotazione'; ok=false
    }; 
    ok=true; 
    if (! ok) alert (errore); 
    return ok; 
} 
</script>
P.S.
primo errore di sintassi evidenziato in rosso!(me ne sono accorto mettendo in oridne il tuo codice..(ci vuole un po'+ di tempo ma ti accorgi subito degli errori!!!)