Hi William,
try logging in through your XXX.artistwebsite.
Navigate to: behind the scenes > General > click on "Custom Domain Name >
Enter the Domain Name you want to use.
An Instructions page will come up with the DNS settings you need to put on your Domain Registrar (where you purchased the domain name, e.g. GoDaddy).
Where it says to add "custom.artistwebsite", make sure to substitute "custom" with your name the way it appears on the webpage.
That's it, it should work now. I just did this for my Mom's site here: http://cynthiamcleanart.com/
Regards,
Ian McLean