I tried to install the command line tools on Mavericks following the steps in this very useful post
> xcode-select --install
but after a
Install them from here. You must be a registered developer.
Other solution is: in xcode click Xcode>Open Developer Tool>More Developer Tools
This should then take you to a link which will require a developer Apple ID sign in. From there, you'll be redirected after authenticating to https://developer.apple.com/download/more/ where you can manually download and install the Command Line Tools.