Custom domains are available on Professional and Business plans. Please note, you must use a subdomain. You cannot add a custom domain using the root. eg. www.example.com Do not include the 'WWW'.
An example of the correct format would-be: proposals.example.com
If you have trouble with any part of the process, be sure to contact us on firstname.lastname@example.org
Go to the DNS settings of http://yourwebsite.com and create a CNAME attribute pointing 'proposals' to 'secure.nusii.com.' (For lots of providers you have to add a dot after .com)
Here is an example of the private DNS settings for michaelkoper.com
'secure.nusii.com' is the subdomain and 'proposals' is my CNAME.
After you have this configured, drop us a line and we'll make sure everything is as it should be.