Any Howto out there seems to assume 2.4.x kernels. 2.6.x should be similar but it doesn't work for me. What kernel and modules do I need for IP MASQ?

The machine in question runs 2.6.8, the kernel has compiled in:

iptables was not used here before. Appearently the iptables command tries 'modprobe ip_tables' which fails. Full output (to any command):
FATAL: Module ip_tables not found.
iptables v1.2.11: can't initialize iptables table `nat': Table does not exist (do you need to insmod?)
Perhaps iptables or your kernel needs to be upgraded.

Installing the ip_tables module won't help (not surprisingly):
FATAL: Error inserting ip_tables (/lib/modules/2.6.8-jk20050516/kernel/net/ipv4/netfilter/ip_tables.ko): Device or resource busy

I want a simple IPMASQ to link Zaurus and dial-up PPP.