What ROM are you using? All current custom kernels and all current ROMs have kernels with netfilter fully enabled. Please note, I said current ROMs. Many ROMs have been abandoned and the latest version isn't current.
If you need a replacement kernel, try P3Droid or ChevyNo1 kernels.
If you're trying to tether an iPad, Wii, or other hardware that doesn't work with Ad-Hoc networking, well, that's not the program's fault.
Finally, are you running the latest version (latest prerelease) of the Google Code app or one from the Market? The ones from the Market are built around the latest "stable" code, and therefore are weeks if not months behind development.