# for users not logged in
add_action('wp_ajax_nopriv_payment_callback_action', 'dixipay_callback_action');
# for users logged in
add_action('wp_ajax_payment_callback_action', 'dixipay_callback_action');
# Your callback
function dixipay_callback_action() {
echo "Its Working!";
}
read more: http://codex.wordpress.org/AJAX_in_Plugins