You have your Amiga hooked up to your ADSL-modem, and want to share that over to your PC?
At the very least, I think you need MiamiDX to do that. But I'm not sure of how to get the Amiga to talk TCP over the null-modem cable.
I remember setting up something similar, only the other way around (Internet-connection to the PC, shared with the Amiga). I might have used NullTCP (can't remember, as it is some time ago). But I also remember setting up a PPP-server on the PC, and then using AmiTCP to connect to that, over PPP.