So, you have an existing MX record and want to add another one. You click on the Add DNS Record button, fill in your values, click add, and you get this message:
Record already exists
To add the record, all you need to do is click on the Edit button located to the right of your existing record.
Depending on the record type, you will either be able to add another record by clicking on the plus icon, or in the case of a TXT record, by entering the new values on a new line.