Xcode is not currently available from the Software Update server
You can download the command line tools for OS X Mavericks manually from here:
For OSX 10.11 or more you can download from here https://developer.apple.com/download/more/.
(The link in the accepted answer doesn't display command line tools for El Capitan (OSX 10.11))
If you are trying this on a latest Mac OS X Mavericks, command line tools come with the Xcode 5.x
So make sure you have installed & updated Xcode to latest
after which make sure Xcode command line tools is pointed correctly using this command
xcode-select -pWhich might show some path like
/Applications/Xcode.app/Contents/Developer
Change the path to correct path using the switch command:
sudo xcode-select --switch /Library/Developer/CommandLineTools/this should help you set it to correct path, after which you can use the same above command -p to check if it is set correctly