You should have MacPorts(http://www.macports.org/?page_id=48) already installed.
~$ sudo port -d selfupdate
Now use MacPorts to download and compile all the dependencies,
~$ sudo port install jpeg libpng libwmf tiff lcms freetype ghostscript
sudo port install tiff -macosx
sudo port install ImageMagick
sudo gem install rmagick
Hope you got RMagick.
