I had this come up. It was because I was using an RVM for ruby instead
of the default OS X ruby. OS X comes with CarbonCore which lets you
access special Apple features like FSEvents. RVM doesn't come with
that, so you have to install it manually. Here's what I did and it
fixed the problem.
Run this in your terminal:
rvm install 1.8.7 --debug --reconfigure -C --enable-shared=yes
Then download and untar: (Don't get the latest, use the one I'm
linking to)
http://sourceforge.net/projects/rubycocoa/files/RubyCocoa/1.0.0/RubyCocoa-1.0.0.tar.gz/download
Open the RubyCocoa directory in terminal and type:
ruby install.rb config --build-universal=yes
ruby install.rb setup
sudo ruby install.rb install
(You may not need to use sudo, but I did)
I hope that works for you. If you're not using RVM I have no idea.