MX records cannot point to an IP address, but there is a workaround. If you only have an IP address for your mail server, you will need to do a two-part setup to get things working.
You will need to be logged in to your account, and go to the dashboard for the domain you want to set up.
Create and save an A record for a subdomain, e.g. mail.mydomain.com, that points to the IP address you have. The subdomain will go in the Hostname field, and you can leave TTL as is at 3600.
Create an MX record for the newly created subdomain, e.g. mail.mydomain.com. If you haven't been provided with a Priority number, use 10. TTL can be left as is at 3600.
For each of the records, click the blue add button to add them to the zone and your list of DNS records for the domain. When you are finished, click the green Save DNS Settings button to save the changes.