I was trying to use a gem named "cfpropertylist" to create an export script on Mac OS X (Lion) with sketchup 8. I had lots of problems, but finally got it working. Others using Mac may be interested.

I installed the GEM to the standard OS X installed ruby (version 1.8.7). I did "sudo gem install CFPropertyList-2.0.17.gem".
That worked, but messed up the permissions on the files so non-superusers could not read the files. I think this may be because I had a weird umask. I used /bin/find to fix that up.
Then I could require cfpropertylist in an irb session.

I then tried to require it in a sample script I loaded in the sketchup ruby console. But it got a file not found. I found a post about replacing the ruby provided with sketchup with the standard OS X ruby on stack overflow: http://stackoverflow.com/questions/3333163/how-to-update-ruby-in-google-sketchup. That worked even thou I have sketchup 8 (and there was no Headers there originally, but I linked one in anyways).

I also saw posts about modifying LOAD_PATH to include the standard dirs. So I put this at the front of my test script:

osx_stdlibpath = ["/Library/Ruby/Site/1.8", "/Library/Ruby/Site/1.8/powerpc-darwin11.0", "/Library/Ruby/Site/1.8/universal-darwin11.0", "/Library/Ruby/Site", "/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/vendor_ruby/1.8", "/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/vendor_ruby/1.8/universal-darwin11.0", "/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/vendor_ruby", "/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8", "/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/powerpc-darwin11.0", "/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/universal-darwin11.0", "."]

$LOAD_PATH.concat(osx_stdlibpath)
$LOAD_PATH.uniq!

It still did not work.
Finally I figured out that I had to require the stuff for GEM in order for it to find the package.
Guess it makes sense--I think irb was including the GEM loader for me.

So I put this into my script after the code modifying LOAD_PATH:

require 'rubygems'

And finally the require of CFPropertyList worked!
-Derek