you can buy it at the ticket machine on day of arrival. each more person till 5 makes it cheaper.
also good for the train to Hannover after match
Clicking "home" on that page, you get to normal ticket sale.
For lower saxony ticket, you have to search connections by clicking "local trains" on the search site
This ticket is only good for Lower Saxony, Hamburg and Bremen, not all Germany.
See this map
http://www.bahn.de/p_en/view/offers/national/regional/laender-tickets/laender-tickets-overview.shtml