I assume you already read this:
http://www.usr.com/support/9000/9000...e%20modem.html
If it's supposed to work with 2.5, it should work with 2.6 too.
Just a couple of tips:
1. Did you specify the correct path for the kernel sources?
2. Are you sure you installed the kernel sources and not only the headers?
du -s /usr/src/* should answer both questions: the sources dir is in the 200MB range