Goto the page graph explorer and generate a short live access token with extended permissions publish_stream and manage_pages rights
Give permission in the popup box
copy the app_id, app_secret and access_token in the following url:
https://graph.facebook.com/oauth/access_token?
client_id=APP_ID&
client_secret=APP_SECRET&
grant_type=fb_exchange_token&
fb_exchange_token=EXISTING_ACCESS_TOKEN
and get the long lived access token.Now open https://graph.facebook.com/me/accounts?access_token=<long lived token>
now copy the access_token from the page you want.
Happy posting for about 60 days..