The most common issue with getting Freight Extensions to work is usually Invalid Credentials.
The other issue we see is people saying the rates are not the same as their negotiated rates. In this scenario we recommend check your configuration on the Shipping Method side, and then if still an issue get the logs off and send them to your rate carrier itself together with a screenshot of your rates obtained direct from their portal. They should then be able to advise on what the issue is, in most cases its a configuration switch or a misunderstanding around weight, origin, etc.
We have produced a screencast that walks through how to diagnose Freight issues, including getting off logs.
If you are still getting problems then refer to our general troubleshooting guide.