Under "Advanced Options" on the left, choose "directDNS"
In the upper left corner, choose "Domain List"
Under the "Domain" Column near the center of the page, click the name of the domain that you want to host using RunBox
If you're using DirectDNS already, you should see some A records. If you haven't set up mail, you should NOT see any MX records.
Select "MX" form the available "Type" options, don't change the subdomain field, and enter your domain name. A good MX preference value is 10. If you know that you need another value for this, you probably don't need to be reading this guide. Click the "Create Record" button.
Now define a catchall MX record. This is the same as the last step, except that you should put an asterisk (*) in the subdomain box. Click "Create Record".