I am really new to PHP and after building my site i open it and get the following message:
Fatal error: main(): Failed opening required 'db/db.php' (include_path='.:/data/apache/php/mmp_lib') in /data/members/free/tripod/uk/e/m/g/emgage/htdocs/mainfile.php on line 87
Can someone with a bit more experience please tell me what to do?

Check and make sure you did this
http://www.nukemods.com/modules.php?...de&op=sqlsetup

And this
http://www.nukemods.com/modules.php?...&op=configfile

Your error means either you did not setup the config.php file yet or your database has not been created.