Does Plattsburgh have a station where they can check your bag in before boarding bus.

Yes

plattsburg station is basically a convenience store with a dunken donuts attached. The person at the cash can help you get a ticket and you will pay for any extra bags also there. The bus will pick you up from the parking lot area. If your going north you will need to show the driver your passport.

Yes if you go inside the gas station. There is greyhound employees to help you for checking your bag in