This is a contentious issue with IK. Even though in the past it was allowed, the budget feature no longer will pick up a mortgage or loan payment in the budget because it's considered a transfer from one account to another. If you didn't have the mortgage or loan set up in MD, and wanted to budget a payment amount, then no problem. But if you build the mortgage or loan in MD, it's a transfer so they don't pick it up currently. But there are work around methods that several of us use to include loan payments in our budgets. Here is a link to one of those discussions.
Just wanted to chime in as someone who thinks it is absolutely insane that Moneydance cannot properly work mortgage payments. While workarounds others have offered are nice to know, it is crazy that they are needed. How in the world this basic part of budgeting is not only excluded but requests to fix simply ignored is beyond any reasonable persons comprehension.