You will need to use
emCORE, the link given describes installation.
After installation of emCORE, you can update the Rockbox binary from
this link, which will always point to the most current version of Rockbox for the iPod Classic.
It is important to note that any emCORE or Rockbox binaries or instructions given from any location other than freemyipod.org or rockbox.org (for instance, ABI forums) are absolutely unsupported. Instructions there are often incomplete, outdated, or just plain wrong.
It is also worth noting that the iPod Classic port is currently flagged as "unusable" by the Rockbox project, and that while many people including myself use this device as their main DAP you will encounter bugs and/or unexpected behaviour.
[Saint]