ip_tables with DSL-N

Hi,
I got the DSL-N distribution (based on Knoppix) and wantted to use
Masquerading on that computer. It turned out, that the kernel modul
ip_tables was not shipped with the distribution.

Since the kernel 2.6.12 was used, I got the kernel sources and compiled
the modules, took ip_tables.ko, put it in the /lib/modules..../netfilter/
place, run depmod, but when I was trying to load it (modprobe), I got a
message:
something like "Error -1: Kernel modul not in the right format".

Was I doing right? Are the hidden kernel differences?
How can I add kernel modules to an existing kernel?

thanx for help
Markus