// Place this in wp-config
define('ACF_5_KEY','yourkeyhere');

// Set ACF 5 license key on theme activation. Stick in your functions.php or equivalent.
function auto_set_license_keys() {

  if ( !get_option('acf_pro_license') && defined('ACF_5_KEY') ) {

    $save = array(
		'key'	=> ACF_5_KEY,
		'url'	=> home_url()
	);

	$save = maybe_serialize($save);
	$save = base64_encode($save);

    update_option('acf_pro_license', $save);
  }
}
add_action('after_switch_theme', 'auto_set_license_keys');