Unfortunately that guide isn't particularly good - I'm actually not particularly happy with many of the guides and tutorials which relate to devkitPro toolchains.
There's a much better guide in the user contributions section @
http://forums.devkitpro.org/viewtopic.php?f=25&t=68 which should help a bit. It's a little out of date unfortunately and refers to older versions of the toolchain. I've started to write up some details on the wiki at
http://wiki.devkitpro.org/index.php/Get ... /devkitPPC where I'm trying to avoid using particular versions so it doesn't need updated.
Your main issue is likely to be caused by using a GUI tool to extract the archive - most of them have trouble with symbolic links and/or properly extracting files from the tarballs.
In the future we hope to find a way to create an automated installer for both OSX and linux, similar to the Windows installer - this is unfortunately complicated by the differences between distros.