Interest Only Loan-Mac

I don't believe you can use the loan feature in Quicken Mac to create an interest-only loan. However, with one manual step, you can easily accomplish what you want.

First, create a Liability account as a manual account not tied to a financial institution. (Click "My bank is not in the list" at the bottom of the Add Liability Account screen, and then click "Add Manual Account".)

Next, you need to create the amount of the loan. Typically, if you are just receiving the money, you would enter an opening balance of zero when you create the account. Then, you'd enter a transaction to establish the loan and the receipt of cash:
    Payee: "Received from XYZ"
    Increase: Amount received
    Category: "Transfer:[your checking/savings account]"
This will create the liability amount, and put the cash in whatever account you put it in.

Alternatively, if this is an existing loan and you don't want to go back in time to deal with the money you received, you can just enter the amount of the loan as the opening balance of the liability account -- establishing the loan but not the receipt of the funds.

Now here's the one manual part: you need to calculate the amount of the payments. Since there loan is interest-only, you won't have an amortization amount which changes monthly, use a simple calculation base don the interest amount and frequency of payment. (For instance, if your loan is 4% annually, and you're paying monthly, your monthly interest payment is .04 ÷ 12 = .003333 times the amount of the loan.)

Finally, you create a scheduled transaction in Quicken for this monthly payment. Since the loan amount is not changing, the loan account isn't even involved; the payment is a simple transaction in your checking account to pay your lender the calculated amount, with the category being interest expense. Set the schedule of the transaction for the frequency and duration of your loan, and it will pop up every month (or quarter or year).

Essentially, you're only recording a monthly interest payment from your checking account. The loan (liability) account never changes until you pay it off or pay it down.