If you're not using one of the ones on the page, then look around your domain registrar control panel for CNAME records. Add one pointing to www.smugmug.com, and it should work (it might take a few house).
If you can't find it, contact your registrar and ask them to point a CNAME record to www.smugmug.com .
Comments
There are directions for a few main domain registrars here:
https://www.smugmug.com/help/professional-accounts
If you're not using one of the ones on the page, then look around your domain registrar control panel for CNAME records. Add one pointing to www.smugmug.com, and it should work (it might take a few house).
If you can't find it, contact your registrar and ask them to point a CNAME record to www.smugmug.com .