Could not find generator form_builder:install

[Bnaima]

I'm trying to install modules in a browsercms project following this tutorial here: http://guides.browsercms.org/installing_modules.html.
I have rails 3.2.18 and browsercms v3.5.7 installed.

First, I installed the following gem:
>gem install form_builder

When I run this for my project:
>rails g cms:install form_builder

I got this message:
Could not find generator form_builder:install.

However it adds to the gemfile this:
gem "form_builder"

Then I run:

rake db:migrate

and

rails server

I could not find the module in the cms.

What could be the problem.

Many thanks.