It is possible that the php5-fpm service that nginx requires to process php scripts is not running, this will result in a 502 error. You can attempt to restart this with the command:

service php5-fpm restart

When a tutorial or guide says to “uncomment” a line it usually means that the configuration line has a character at the beginning of it which causes it to be seen as a comment and not a setting to use. Often this will be the # character.