The Purchases window should be displayed. (To find the Purchases window)
If the line on which you want to insert the subtotal is empty -- no transaction is entered on the line, and it's not a blank, header or subtotal line -- click in the line. (The subtotal will be inserted here.)
If you click on a line where there is a transaction, blank, header, or subtotal line entered, the subtotal will appear above that line.