Your site must be TURNED ON first before you can buy or assign a domain name.
If you cannot see how to set or buy Domain Name it is probably because your site is OFFLINE.
(To see if your site is ONLINE / LIVE see the COLORED DOT to the left of the SITE on the MANAGE SITES page. If it is GREEN then it is LIVE!)
As you ACTIVATE your site (or if it is already ACTIVE) then you will be able to purchase a Domain Name.
For an ACTIVE site, the "domain name" icon is in the EDIT page.