This may help some people. I'm running an Athlon 64 X2 dual core machine and finally got Cool 'n' Quiet working (not sure about the 'Quiet' bit but that's probably because of my cheapo PSU!)
I installed the K8 SMP kernel using Synaptic and followed the instructions in this howto. However I, like others, got the following error:
I was pretty sure that I had enabled the Cool'n'Quiet in the BIOS of my motherboard (ASUS A8V-VM). However, being a bit of a noob, I had only enabled 'Power Management' thinking this was the name for Cool'n'Quiet.
# modprobe powernow-k8
FATAL: Error inserting powernow_k8 (/lib/modules/2.6.15-26-k7/kernel/arch/i386/kernel/cpu/cpufreq/powernow-k8.ko): No such device
After rooting around in the BIOS interface, I found the real Cool'n'Quiet option, so I enabled it and this solved all the problems.
Incidentally, because I have two CPUs, I have a line for each in my /etc/sysfs.conf. I assume this is correct.