I just checked with our Director of Business Development (who facilitated the integration with ZD). He's fairly certain that the integration should work while you're on a free trial, but if you're on a free plan it will not work. I'll try to reach out to our Zendesk contact for 100% certainty, but it should work on a free trial.
That said, do you happen to know if you used "https://" when entering in the credentials? Get Satisfaction is now on SSL, and I've seen that error message occur if you're not using https://
Give that a shot, and let me know if you're still having difficulties. In the meantime I'll look around and see if I can find any other information.