I recently added my rvm (ruby version manager) installed rubies as platform for NetBeans and it picked the directory of the gems wrongly. After fixing them it worked like a charm.

In the attached screenshot it is shown how NetBeans picks them automatically. Running gem environment shows what are the correct ones and setting NetBeans to it just works. It would be nice if it worked out of the box:

$ gem environment
RubyGems Environment:
  - RUBYGEMS VERSION: 1.3.6
  - RUBY VERSION: 1.9.1 (2010-01-10 patchlevel 378) [i386-darwin10.2.0]
  - INSTALLATION DIRECTORY: /Users/pupeno/.rvm/gems/ruby-1.9.1-p378
  - RUBY EXECUTABLE: /Users/pupeno/.rvm/rubies/ruby-1.9.1-p378/bin/ruby
  - EXECUTABLE DIRECTORY: /Users/pupeno/.rvm/gems/ruby-1.9.1-p378/bin
  - RUBYGEMS PLATFORMS:
    - ruby
    - x86-darwin-10
  - GEM PATHS:
     - /Users/pupeno/.rvm/gems/ruby-1.9.1-p378
     - /Users/pupeno/.rvm/gems/ruby-1.9.1-p378%global
  - GEM CONFIGURATION:
     - :update_sources => true
     - :verbose => true
     - :benchmark => false
     - :backtrace => false
     - :bulk_threshold => 1000
  - REMOTE SOURCES:
     - http://rubygems.org/