I answered my own question. It looks like the cash flow forecasting report is taking current receivables, current bank account balances, and current vendor payables due to calculate cash flows. Therefore, in order to add payroll to the mix I would need to enter payroll into the system for the cash flows forecast report to show these items, as well.
So... the next question is: does QB have a report that will take past cash flow debits and use them with current receivables to calculate future cash flow? Anyone?