Taken from the Koala README, this will post to a users wall:
@graph.put_object("me", "feed", :message => "I am writing on my wall!")
So, the following will post to the Wall of a page (in your context):
graph = Koala::Facebook::GraphAPI.new(self.token)
graph.put_object(page.id, "feed", :message => "I am writing on a page wall!")
Ensure, that the token you use is a user token (if you want to post as a user) or a page token if you want to post as the page itself.
More to read about Graph(Page): http://developers.facebook.com/docs/reference/api/page/
and Koala: https://github.com/arsduo/koala
EDIT: due to comment
To get Pages of a user
@graph.get_connections("me", "accounts")
Documentation: http://developers.facebook.com/docs/reference/api/user/