There are a couple of things wrong. First, mysql_query takes a string, so your entire SQL statement should be in quotes. Second, you have the $_POST variables inside single quotes, so php won't evaluate them (you get then name of the variable instead of the value). Third, I believe "users" is a reserved word; mysql might let you create a table with that name but it could cause problems with other queries.
Anyway, try something like this (untested):
$addNew = mysql_query("
INSERT into users_table (username,password,email) VALUES ('" . $_POST['username'] . "', '" .
md5($_POST['password']) . "', '" . $_POST['email'] . "' ");
