Login to the domain management console
-MouseOver the DOMAINS tab and click My Domains
-Select the domain you want to modify
-On the Details page click ‘Total DNS Control and MX Records’
Here you can modify existing MX record OR add a new MX record for a particular domain.
I think for you as for any customer of them that should not be a problem at all to call them and ask for the help. Why are you trying to solve everything by yourself?
In any case I’m happy to hear you have fixed everything with them though.