For the relevant domain, under the "DNS" column, select "Manage."
Use the "Add a DNS Resource Record" action to add each MX record given to you by your email provider:
Leave the "Name" field blank to apply forwarding to your whole domain.
Select the "MX" type.
Enter both the MX preference and the destination host in the "Data" field, separated by a space. Be sure to enter a "trailing dot" (a period) at the end of the destination hostname. (Example: "0 mail.example.com.")