I am having the same problem, but I am getting a different error.
postid = FBserv.Api.Stream.Publish("Hello World Posted from my app", null, null, null, pageID);
Error: Facebook.Utility.FacebookException: A session key must be specified when request is signed with a session secret
Looking into the source for the facebook dll it seems the session key is only added if the uid is 0. This creates a problem when trying to write to a wall as the wall. I tried modifing the facebook dll source code to test my theory, but I can't
load the project properly through vs2008 (not sure why???).
The permission error message is due to the page not having rights to publish to itself (seems stupid but this is the way facebook set it up). When logging in using the connect URL make sure to request offline access and publish stream rights, this
will bring up a rights page after logging in. The first page will have a drop down box make sure to check all the pages in the drop down box. This should fix the permission error.