MX records cannot point to an IP address, but there is a way to work around that. Read on!
If you only have an IP address for your mail server, you will need to do a two-part setup to get things working.
First, create and save an A record for a subdomain (e.g. mail.mydomain.com) pointing to the IP address you have. The subdomain will go in the Hostname field, and you can leave TTL as is.
Then 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.
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, and then click the green Save DNS Settings button to save the changes.