Hi,
That error code means 'connection timed out.' It could really be that the server is not responding (I just checked the major TopoFusion servers and they are serving tiles quickly).
My first guess, though, is that you need to allow TopoFusion to access port 80 in your firewall software. Is it your home computer? If not you may need to check with your IT guys about allowing the application access.
If that doesn't help, let me know and we can dig deeper into it.