To use an IP address for your MX records requires a three part process.
Create two subdomains (such as mx00.domain.com and mx01.domain.com) since two addresses are required for an MX record.
Assign the IP address(es) for your mail server(s) to each subdomain. The steps described for domains are exactly the same as for subdomains.
Use the two subdomains created that point to the IP address(es) of your mail server(s) as the MX records for a domain.