Sorry my fault you will need to add a where clause to this select query stating which actual username you are searching for otherwise you will ALWAYS get records back - and hence always get the message you are getting!
PHP Code:
$sql_usermysql_query("SELECT mUsername from Members where mUsername='".$_POST['Username']."'") or die(mysql_error()); 
try that first and then we will sort the rest - and yeah sorry I confused you

and for the now() part you would write mJoindate=now() in your insert query