I am installing compass on mac. when I run gem install compass command on terminal I got following output

I already have ruby ruby 1.8.7 (2012-02-08 patchlevel 358) [universal-darwin11.0] installed. I got this output after issuing ruby -v command on terminal.
How can fix above problems or how can I install compass successfully?
As far as I know you should be a root to install gems when using ruby from system.
It is a very good idea to run ruby with rvm or similar tools instead of running
system.