the record set should look something like this
$colname_userexist = "-1";
if (isset($_POST['UserEmail'])) {
$colname_userexist = (get_magic_quotes_gpc()) ? $_POST['UserEmail'] : addslashes($_POST['UserEmail']);
}
mysql_select_db($database_MYDB, $MYDB);
$query_userexist = sprintf("SELECT UserID, UserEmail FROM User_db WHERE UserEmail = %s", GetSQLValueString($colname_userexist, "text"));
$userexist = mysql_query($query_userexist, $sofiaslu) or die(mysql_error());
$row_userexist = mysql_fetch_assoc($userexist);
$totalRows_userexist = mysql_num_rows($userexist);
in other words you add a new recordset the normal way dreamweaver adds them and on filter select the field you want to check for unique names, select from Form Field, and on the next box the value of it then you will need to use the Validation toolkit server side.
With validation toolkit you can add the total row count for that query, $totalRows_userexist and select validation type: Number
When you insert it it will ask you the number values, the Format should be international it didnt work for me with the other formats, then on minimum put -1 and maximum 0, uncheck allow blank if its checked, click ok and add your msg
Hope this helps i miss the easy ways to do stuff also with addt some things with webassist take a lot more time consuming but you can control a bit more how things should be.