Struggling to set up domain mapping


I am trying to set up domain mapping and I don't fully understand what I am being asked to do:

Please copy the sunrise.php to /home/cluster-sites/1399/z/ and uncomment the SUNRISE setting in the /home/cluster-sites/1399/z/ file


If you've not already added define('SUNRISE', 'on'); then please do so. If you added the constant be sure to uncomment this line: //define('SUNRISE', 'on'); in the wp-config.php file.

I have looked in the wp-config.php file and this:

//define('SUNRISE', 'on');

isn't there.

I could give you the ftp details if it would help you figure this out for me, I'm sure its simple but It also doesn't help that I only have limited php knowledge.