This is a how to install Middleman mac guide on Mountain Lion OSX. This took me a bit of figuring out to get working – partially because of unavailable libraries and scripts.
I eventually got there in the end and have compiled these set of commands and instructions you will need to run.

At the time or writing, this has been tested and working on Apple Mountain Lion OSX 10.8.2.
Install the latest version of XCode with command line tools

Install Middleman Mac instructions

Install brew

http://mxcl.github.com/homebrew/
ruby -e "$(curl -fsSL https://raw.github.com/mxcl/homebrew/go)"

Instal GNU Tar (if it’s not available)
brew install gnu-tar

Install RVM
curl -L https://get.rvm.io | bash -s stable
rvm requirements run force

Update GEM to the latest version
sudo gem update --system

Install and run Middleman
http://middlemanapp.com/

gem install middleman
gem install middleman-more
middleman

Documentation on how to use Middle is available at http://middlemanapp.com/getting-started/

Note that with Mac OS X, it comes pre-packaged with Ruby and Rubygems, however, Middleman has dependencies that needs to be compiled during installation and on OSX that requires Xcode. You can install the latest version of Xcode via the Mac App Store. However, If you have a free Apple Developer account, you can also just install Command Line Tools for Xcode from the developer’s downloads page.