I am unable to get a valid access_token too.
When trying to call "/access_token" with the oauth_token I got from "/request_token" I get "oauth_problem=signature_invalid".
After a while I found out that if I supply "blabla" as oauth_token, I get "oauth_problem=token_rejected"... this happens with every string I try, except for the oauth_token I got from "/request_token"
Perhaps this has anything todo with my application not being published? Even then, I need it to work so that I can develop my application (and flow) in a normal way.