This is a series of posts leading toward a complete installation of NEURON neural simulation environment on OS X. All of this information is relevant for other purposes as well.
There are many places that have instructions on how to install XCode. For Lion and Mountain Lion, you go to the Mac App Store and download and install the XCode installer. If you previously had a version of XCode that installed in a directory called /Developer, excellent directions on how to use xcode-select are found on MacPorts.
Command line tools are now treated as a separate download. There are (at least) two ways of getting them.
1. Open Xcode. Go to the Xcode application menu and select Preferences. From there, go to Downloads and select the button for Command Line Tools.
2. Go to the Apple Developer website. You’ll need to be a free developer member to use this website. From there you will go to the Mac Developer downloads section, and you’ll have to find the relevant package for Command Line Tools for your OS. I’ve not tested whether or not you can install these without Xcode, but I’ve also heard that you may be able to either directly or with a little Homebrew hackery (note: I still recommend and use MacPorts to both Fink and Homebrew, but they all have strengths and weaknesses).