Hi,
First of all, have you got the latest stable release of TelldusCenter (2.0.4)? And it is not already activated?
A probable cause of the problem is that your SSL certificates are not up to date. If you, using Internet Explorer, log in at live.telldus.com and restart your computer after that, your certificates will be updated if they were too old.
If that's not the case, it may be due to that your TelldusCenter cannot contact our servers. Are you for example blocking any outgoing ports in your firewall/router?