They have a handy auto install: http://www.rockbox.org/download/Sponsored Links
if it does ask for your version and there is no 5.5g, use 5g. worked for me.
Also, can anyone explain in detail how to do this? my 5.5g ipod is setup nicely with a boot loader, new classic firmware over the apple one, rockbox, and ipod linux. So I want to make sure what im doing is correct rather than re installing everything.