We assume that you have fresh OS X 10.8 (Mountain Lion) installation to start with.
First you should install xcode (i.e. from AppStore or dmg from Apple Developer portal).
There some additional xcode packages (command line tools and iOS simulators) that you have to install from xcode. In order to do this open xcode preferences (or press cmd + ,) and select downloads.
You have to accept xcodebuild license. This is a necessary step to be able to run xcode build commands.
Run the followng command:
Perform the steps described in Jenkins OSX installation