This is from the FAQ and explains why users don't immediately have access once they reach the required number of posts or length of membership:
Once you reach the required number of posts and length of membership, you may not immediately be able to post in these forums. The privilege is added automatically when the forum system does routine processing, so check again tomorrow. The delay is normal, so please do not contact us to report it as a problem on the same day that you first meet the requirements.