Here's what I did:
1. gem install calendar_date_select
2. Then in my environment.rb I put: config.gem "calendar_date_select"
3. And in my template file: <%= javascript_include_tag "prototype" %> <%= calendar_date_select_includes %>
But still an error posts like this : undefined local variable or method `calendar_date_select_includes' for #<ActionView::Base: 0x7bfd920>
My ruby is 1.9.2 rails 2.3.5 gems 1.5.2