you'll notice that if you reset the cable modem and the AP then the AP will be given an IP address, but it wont be a genuine one connected to the internet... it'll be on the cable companys internal network, and any webpage you visit will automatically forward you to the signup screen (where it adds the mac address to your account)

try this... in this order:

1) plug cable modem into access point
2) reboot cable modem
3) reboot access point
3) reboot PC (the PC by the way must be set to use the AP as its DHCP server)

then try getting on a web page n HOPEFULLY you should see the cable companies signon screen...