Obtaining an App Access Token
To obtain an App Access Token, invoke the following HTTP GET request:
GET https://graph.facebook.com/oauth/access_token?
client_id=YOUR_APP_ID
&client_secret=YOUR_APP_SECRET
&grant_type=client_credentials
The API will respond with a query-string formatted string of the form:
access_token=YOUR_APP_ID|YOUR_APP_ACCESS_TOKEN
Reference: http://developers.facebook.com/docs/opengraph/howtos/publishing-with-app-token/