> The help page linked on previous answers
> (http://httrack.kauler.com/help/Authentication)
> isn't anymore avaiable. 

Hopefully that will come back soon.

You logout of the site with browser, then get to the log in page.
Use the capture url button. Httrack will give you a IP address and port.
Modify the browser's proxy address and log in. You'll get a message page from
httrack instead of the site.
Change the browser's proxy settings back. and start the mirror.