Hi Valentine,
The servers seem to be responsive from here. Perhaps it is a firewall issue on your end.I have been trying to load some METAR data via IDV. However, I am unable to connect to *any* of the data servers including add.ucar.edu. I am able to ping the server(s), but couldn't connect via IDV.
See the support entry: http://my.unidata.ucar.edu/content/support/help/MailArchives/idv/msg00483.html -Jeff