I installed rails two days ago and it worked great, why the sudden disappearance?
!  Invalid path.
!  Syntax is: [email protected]:<app>.git where <app> is your app's name.

ERROR:  Error installing rails:
actionpack requires rack (~> 1.3.5, runtime)

$ rails
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems.rb:230:in `activate': can't activate rack (~> 1.3.5, runtime) for ["actionpack-3.1.3", "rails-3.1.3"], already activated rack-1.4.0 for ["rack-cache-1.1", "actionpack-3.1.3", "rails-3.1.3"] (Gem::LoadError)
    from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems.rb:246:in `activate'
    from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems.rb:245:in `each'
    from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems.rb:245:in `activate'
    from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems.rb:246:in `activate'
    from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems.rb:245:in `each'
    from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems.rb:245:in `activate'
    from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems.rb:1056:in `gem'
    from /usr/bin/rails:18

cd your_app_directory
bundle install
gem install heroku
git add remote heroku [email protected]:<YOUR_HEROKU_APP_NAME>.git
heroku status --app <YOUR_HEROKU_APP_NAME>