...
$userquery = mysql_query("SELECT  kategorie_Nummer FROM $prefix"."_users WHERE user= '$sel'") or die(mysql_error());
while($line = mysql_fetch_object($userquery)) {
....

echo '<td width="170" bgcolor="#e7e7e7">Kategorie</td></td>';?>
        <td width="230" bgcolor="#ffffff">
            <?PHP
            $types = mysql_query ("SELECT  bezeichnung FROM $prefix"."_kategorie ")or die(mysql_error());
        echo "<select name='types'>";
        while ($cat = mysql_fetch_array($types)) {
        echo "<option>".$cat[bezeichnung]."</option>";}
        echo "</select>";
        ?>
        </td>