Rockbox can be found here:
http://www.rockbox.org
Installation guideline is here:
http://www.rockbox.org/twiki/bin/vie...RockboxInstall
You will need to run a patch program on a flash file from iriver and flash the resulting file using the iriver firmware. Make sure you do this step exactly as written down because this is the only step that can break your player. Then again, every time you flash your player with official firmware holds the same risk.
And you also need rockbox:
http://www.rockbox.org/daily.shtml
If you need more info, feel free to join the #rockbox IRC channel, there's a webclient for easy connection (
http://www.rockbox.org/irc/cgiirc/irc.cgi)