kernel --> where --> syssrc.tgz (newbie)

i want to prove my first kernel update, now 
i have downloaded the completed iso cdrom from
netbsd 1.6.1 and i banned on a cd but after a scan 
on this i dont find this file "syssrc.tgz"

i have read it that this i need for a kernel update 
to make the modifications,

please iam a newbie on my NETBsd but its goes every 
day better to work with a good system

thanks in advance 
Maurizio