You can call to The Ainu Museum to book it in advance.
The day of the show you booked, you pay at the entrance booth before the show.
The Ainu Museum has Website and E-mail address.
You may ask about POROTOKOTAN by e-mail.
The Ainu Museum
http://www.ainu-museum.or.jp/
TEL # 0144-82-3914
museum@ainu-museum.or.jpYou can also buy a ticket at these convenience stores (Lowson, Seven-Eleven, Family-Mart). They have special booking machine.
There are two different ticket plans.
One is POROTO plan(2,000yen). Another is PIRIKA plan(Including Dinner ticket/4.000yen).
Both plans have the guide tour (maybe only Japanese guide) of museum before the show.
POROTOKOTAN Web site (Japanese only)
http://www.porotokotan.jp/