[Rails] shoulda installation troubles

I cannot manage to have shoulda working.
I'm using rvm with rails 3.1 and ruby 1.9.2
I added
group :test do
gem "shoulda"
end

to the Gemfile, and then run bundle install, thus the gem is installed,
and it is featured in the list running gem list --local.

but whenever I add something like
class ModelTest < Test::Unit::TestCase
should_not allow_value("blah").for(:email)
end
in a testfile, running that test with ruby -Itest
test/unit/model_test.rb
I get an error undefined method `allow_value' for ModelTest:Class
Am I missing something?
should I manually require shoulda? where and how?
thanks