The coolant will not flow past the overflow tank?

1997 Plymouth Grand Voyager mileage: 76,000. I drained and flushed the coolant from the van. When I attempted to add fresh coolant, the fluid won't flow past the overflow tank. My Haynes manual says that the engine vent plug must be loosened first. I can't find it. The plug is not where it is shown in the manual diagram. Where can I find this plug? Is there any other way to get the coolant past the overflow tank into the radiator and engine? I started the engine briefly, and that did not help.