7z compression

[Curt]

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