A packet analyzer seems to show that it's actually trying 
to look up (and presumably connect to)  "@www.host.com" 
rather than "www.host.com"

I've tried both entering

<http://user:pass@www.host.com/page.html>

directly into the Web Addresses textbox, and tried using 
the Add Url dialog, by entering

www.host.com     for URL
user    and   pass     respectively

which adds an entry just like the original into the Web 
Addresses textbox.

Thanks