well there is two ways you can do this:
easiest way to is clone you MAC address of the orignal computer they install the adsl on.
If it was on your Mac your MAC address by going to perefences, network, built-in ethernet, ethernet, Then the Ethernet ID is MAC address.
write that thing down.
now goto,
http://support.dlink.com/
get your manual and lookup how to do cloning. in a brief summary:
basically goto
http://192.168.0.1 and login. if you stuck here refer to the manual. it will walk you through the basic admin process. anyways once you there click on "WAN" on the left column. Type in the ethernet ID that you wrote down where it says MAC Address. remember this has to be the orignal computer that you had registered. after the changes are saved, unplug your router for about 1 min. allow the server on the ISP end to disconnect you totally from the system. this is important. after that plug back, hope fully that will be it. all the garbage above did is make the ISP think it was your old computer. it really isn't hard, it just took a lot of typing.
OR
just call them and say you want to register a different computer.
the first is actually better, depending on the policy they might not allow routers on their network or they will not give you technicall support. it depends on their policies.
