Here is a picture of the wicket gate at the station. If you are using a paper ticket, insert it into the first slot you see.
Planning on making at least one round-trip to Haneda Airport from Hamatsucho? Then get the Tokyo Monorail one-day pass, which is good for unlimited rides on the monorail within the same day of purchase.
For more information, see this page (only in Japanese): http://www.tokyo-monorail.co.jp/tickets/value/ensenosanpo.html