Are you using all lowercase in the channel name?
This link would show chat history, but not allow you to send messages: http://twitch.tv/ExampleChannel/chat
This link would allow you to see messages and send messages: http://twitch.tv/examplechannel/chat
Same effect as if you were on TMI (Twitch Messaging Interface)
I usually suggest emedding chat with: http://twitch.tv/chat/embed?channel=Matt_Thomas&popout_chat=true
As this link will redirect you to http://twitch.tv/matt_thomas/chat ( it will fix the case, but you can just as easily use lowercase )