If you want to build a website using your custom domain name, this requires you to upgrade to Pro.
If you are not building a website with your domain name and you just want to buy an email address, you can do that as a free member without upgrading.
You have your domain already from GoDaddy. To get an email using this domain name, go to your account settings (click on your user name at the top right corner of Bookmark, then click Account Settings in the profile window). There is a tab in Account settings called My Services.
Inside My Services you will see the option to Connect Email to your Domain.
Since you're using a domain you purchased from GoDaddy, you will need to change some Domain settings in your GoDaddy account to complete the setup of the email address (if you purchase a domain from Bookmark, we do this set up for you). Instructions will be sent to your email once you make the purchase on how to setup your email address. There is also a user guide in the Help Center.