Run Miami Init and let it do the config for you.
Then Run Miami and import from miami Init.
Then like Kev said reg your Modem MAC at the reg site. You'll need your account number etc handy.
However, The OS3.9 / Full latest Aweb was the only Amiga browser that **worked** with the ntl regsite. IBrowse 2.3 might 2.2 didnt
You might be better off using a PC to set up the account. Then copy your details accross.
Dont ring the helpline there too busy posting on amiga.org ;-)
-edit-
If the modem has been plugged into another computer before you plug it into your amiga, you will need to power down the modem for 30 seconds (not standyby unplug) before you plug it into your amiga. Designed to only work with only 1 ethernet card MAC address.