It sounds like you don't have a 5th generation iPod, but rather the newer "Generation 5.5"/ "Late 2006"/ "40% brighter screen" iPod.
Rockbox does not currently work on this model officially.
There is a forum thread here in the "New Ports" section on a way to load Rockbox on a gen 5.5, but the process is not for the faint of heart, and the usability appears questionable.
That said, it is being worked on, and no there is no time line for when support will be ready.