I believe the network is set up to prevent people plugging in routers, running DHCP servers, etc. Also, running your PCs behind a router would interfere with your access to the network - no LAN game browsers for a start.
As for how to use it the proxy..
The PC with 3G connected to it ("3G-PC") needs to have AnalogX Proxy installed. Find out the IP addres of 3G-PC. Make sure Proxy is running - it'll have a little green icon in the system tray if it is. If it's red, there's something wrong. Have a look in options and try and figure out what the problem is.
On the other PC ("C-PC"), programs need to be configured to use a proxy. To do this in Internet Explorer (IE's settings carry across to a number of other programs, such as MSN Messenger, but not everything), go to Tools -> Internet Options -> Connections -> LAN Settings. Tick both the "Use a proxy server" and "bypass proxy for local addresses" box. Put in the IP address of 3G-PC, and for the port, have a look in the handy-dandy-analogx-proxy-readme-file - I can't remember what port it uses off the top of my head. It's something obscure, but you're looking for the HTTP port. Stick that port in to the port box of IE's LAN settings, click ok a few times, and it should work... Probably.
Firefox uses a similar method.
QUOTE (xevious)
you may have to come help us on the day,
I may or may not have time to help with that. If you do come and seek help from me, you'll be quite a way down the priority list. You'd be better to ask the people around you and see if they know how to do it. =)