Ruby cannot find gem (populator) even though it's installed

I am trying to work with the Populator gem. I just installed it, but Ruby is unable to load it. I have only one Ruby installation on my machine and it's Ruby 1.8.7. What can I do?

This is the gem that doesn't get loaded.

Edit: Here is the trace out. I should have included this earlier.

Freenaut:bio-clinical newuser$ rake db:populate --trace
(in /Users/newuser/Dr开发者_Go百科opbox/Sites/rails/bio-clinical)
** Invoke db:populate (first_time)
** Invoke environment (first_time)
** Execute environment
** Execute db:populate
rake aborted!
no such file to load -- populator
/Users/newuser/.gem/ruby/1.8/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:239:in `require'
/Users/newuser/.gem/ruby/1.8/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:239:in `require'
/Users/newuser/.gem/ruby/1.8/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:225:in `load_dependency'
/Users/newuser/.gem/ruby/1.8/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:591:in `new_constants_in'
/Users/newuser/.gem/ruby/1.8/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:225:in `load_dependency'
/Users/newuser/.gem/ruby/1.8/gems/activesupport-3.0.0/lib/active_support/dependencies.rb:239:in `require'

To date the Populate plugin just does not support Rails 3. See the GitHub site for current status.

This seems to be a really handy plugin, hopefully there will be Rails 3 support soon.

Are you working with Rails or just Ruby?

Did you use

require 'rubygems'

before require 'populator'?