If you use a proxy for browsing the following "cut 
and paste" from Fred Cohen's excellent users guide 
should get you going.

eg: to dowload www.cnn.com i use:
httrack -P 10.254.0.34:3128 www.cnn.com

where 10.254.0.34 is my proxy at port 3128.

Now the "cut and paste":
 Many users use a proxy for many of their functions. 
This is a key component in many firewalls, but it is 
also commonly used for anonymizing access and for 
exploiting higher speed communications at a remote 
server. Proxy options:
  P  proxy use (-P proxy:port or -P 
user:pass@proxy:port)
 %f *use proxy for ftp (f0 don't use)

 If you are using a standard proxy that doesn't 
require a user ID and password, you would do 
something like this: httrack <http://www.shoesizes.com> 
-O /tmp/shoesizes -P proxy.www.all.net:8080 

 In this case, we have asusmed that proxy.www.all.net 
is the host that does the proxy service and that it 
uses port 8080 for this service. In some cases you 
will have to ask your network or firewall 
administrator for these details, however, in most 
cases they should be the same as the options used in 
your web browser. 

 In some cases, a user ID and password are required 
for the proxy server. This is common in corporate 
environments where only authorized users may access 
the Internet. httrack <http://www.shoesizes.com> -O 
/tmp/shoesizes -P fc:password@proxy.www.all.net:8080 

 In this case, the user ID 'fc' and the password 
'password' are used on proxy.www.all.net port 8080. 
Again, your network or firewall administrator can be 
most helpful in addressing the specifics for your 
environment.