Once logged in, click on the Websites tab.
Click on the domain you'd wish to add or update a DNS record for.
Click the "DNS" Tab.
Then click the "DNS Records" link.
You'll see the list of existing DNS records.
NOTE: You can't delete the default entries that are created by the system...i.e. MX, NS or the A records. If you need to change the A record for the main site, you'll need to deactivate the default records, then you can add a new A or MX records.
To add a new record, click the "Add New DNS Record" link.
From the DNS Record type section, select what type of record you want to add and fill in the information for the new entry.