Has your domain resolved to our server yet? After you change your nameservers it can take up to 72 hours for the nameserver changes to propagate around the world, if you've just changed your nameservers or maybe you haven't changed them yet then you can put the IP address of the server as the FTP host.
Are you on a network or do you have a firewall that could be blocking the FTP connection? If so, please reconfigure your firewall or use a standalone PC.
Our firewall may also block your FTP connection in some circumstances, in that case please select 'Use Passive FTP' in your Design or FTP software.