[Radiant] can't install

[Tom Cloyd]

I'm trying to follow the instructions at http://radiantcms.org/download/ 
- and I'm a weekend student of Ruby, newly migrated to Kubuntu Linux. 
I'm almost certainly doing something stupid I cannot see, so...

Here's what happened when I tried the gem install:

tom at tomcloyd-desktop:~$ gem install --include-dependencies radiant
Bulk updating Gem source index for: http://gems.rubyforge.org
ERROR:  While executing gem ... (Gem::GemNotFoundException)
    Could not find radiant (> 0) in any repository
tom at tomcloyd-desktop:~$ gem install --include-dependencies radiant-0.6.4
Bulk updating Gem source index for: http://gems.rubyforge.org
ERROR:  While executing gem ... (Gem::GemNotFoundException)
    Could not find radiant-0.6.4 (> 0) in any repository
tom at tomcloyd-desktop:~$

I don't know what the problem is - and someone give me a clue?

Thanks, in advance.

Tom