For whatever it's worth ... I would not bother attempting to activate any loan or mortgage account for downloading. (Not discussing LOC or HELOC accounts here)
An online-connected loan or mortgage account does NOT have a transaction register. All data shown in the account come from whatever information the bank downloads to you ... if this process works at all.
As a result of being connected, the scheduled payment transaction reminder cannot transfer the amount of principal paid into the (non existent) account register and must use a category, usually something like Loan:Principal, instead.
Since you're attempting to set up a loan account, create a "manual" loan account that is not connected to the bank for downloading. When you start the Add Account process select the account type and on the next screen click the "manual loan account" text link at the bottom of the screen. Perform the loan setup as of the next payment due and set up to use a Scheduled Reminder for the monthly payment from your Checking account. It will correctly transfer the amount of principal (and optional additional principal) from Checking to your loan account register in Quicken.
Does the monthly payment include "Escrow Withholdings" (taxes, insurance) withheld and paid for by the mortgage company? If so, before you begin to set up the loan, set up an "Escrow" account (as Asset type account), so that the Scheduled Reminder can include the escrow amount as transfer to the Escrow account register.
Now all you have to worry about is to make the monthly payment on time. I recommend authorizing the mortgage company to withdraw payment plus escrow automatically from your checking account every month, on the Due Date.
