When you buy it you have to enter the nameservers (get them from your host) and that will direct it to your site which is all we did to move this domain from lunarpages to pc core. You might also need to park it in the cpanel as you should have some domain options their.
When you register you should have a log in area where you have a little control panel i think where you will be able to enter the nameserver info. It doesn't require any info from your config.php file as that just connects the forum to the database. I believe the only info you need to enter is the nameservers ns1.whatever.com and ns2.whatever.com or what ever the nameserver are for your host.
Do they allow you to use custom nameserver? If so, point them to the ones your host gave you. What control panel are you using for your site? cPanel, Direct admin, Hsphere...? If you are using cPanel, login and set up an addon domain, and point the domain to the /forum folder.
If you have MSN or AIM, you can add me, and I can walk you through it the steps.
Apologies for the double post being slightly early, although this is important.
I have now added topgearforums.co.uk as a parked domain, and it works ~cheesey_grin~ - however, when i click it and go the forum, the address bar changes to topgear.rea-lhost.uni.cc/forum
anybody know a way of changing this in the c-panel?