7z compression

Just downloaded a Gentoo based livecd that has XGL running on both gnome and kde.... Unfortunately, I can't seem to extract the iso image with either Ark, File Roller or Karchiver. I installed p7zip and using command line 7za I keep getting unsupported file type errors. I am unfamiliar with 7za and p7zip... Can someone offer info on how to extract this file? file name=RR64-Linux-3.0b1.iso.7z Thanks