Look for the "Dynadot Hosting" box at the bottom of the page. Look for the "Edit Site" column and click the "[edit website]" link to access your web hosting.
A new browser window should open displaying the statistics for your web hosting plan. To create email addresses for your domain email, click the "Admin" section link and then click the "Mailbox Admin" subsection link.
Enter in the email address you want to create, and then click the "Create New Mailbox" button to create it.