To get a long-lived access token you need to follow those steps:
Create an Application
Create a Page (your account need to be "administrator" of the page)
Associate the application to the Page (the same way you do it when
you want to add a Page Tab to a Page)
Get a short-lived access token with the permission "manage_pages"
associated to your Application.
https://graph.facebook.com/oauth/authorize?client_id=APP_ID&scope=manage_pages&redirect_uri=http://www.facebook.com/connect/login_success.html
then
https://graph.facebook.com/oauth/access_token?client_id=APP_ID&redirect_uri=http://www.facebook.com/connect/login_success.html&client_secret=APP_SECRET&code=CODE_FROM_PREVIOUS_REQUEST
Using the Graph API Explorer with the request /me/accounts you can
see the access tokens for each Pages that you are administrator.
The problem is that those access token are short-lived.
Convert your short-lived access token to a long-lived (extending
access token):
https://graph.facebook.com/oauth/access_token?client_id=_APP_ID_&client_secret=_APP_SECRET_&grant_type=fb_exchange_token&fb_exchange_token=_ACCESS_TOKEN_ON_STEP_4_
You can now test your new access token with the Access Token Debugger.