Having MRINs more accessible in RootsMagic is an enhancement request in our tracking system. The reason this has not been implemented is due to our concern at RootsMagic that MRINs (like RINs) can change when you export your database into a GEDCOM or Merge. If you tie everything to a MRIN or RIN there is a chance you could screw your entire filing system up. We prefer that users do not become dependent on computer generated numbers. The preference is to use the Reference Number fact to create unique numbers for each individual in your database.
To display the reference number go to Tools>File Options>Number to display after name - select in the drop down box "Reference Number"(REFN).
Since I use the REFN fact to keep track of something different I created a new family fact called "Family Ref#" and went off the MRIN list (Report>Lists>Marriage List) to add the fact and number to every couple. Downside you can't view that number unless you are in the person's edit screen. Plus is I don't have to generate the MRIN list to see it.
If you use the REFN fact you need to add the same number to both individuals in a couple.
If you use your own "Family Ref#" and create it as a family fact you only have to add it to one individual in the couple.