For Xcode 8 you gotta download a package named Additional Tools for Xcode 8
For other versions (8.1, 8.2) get the package here
Double click and open the dmg
and go to Hardware
directory. Double click on Network Link Conditioner.prefPane
.
Click on install
Now Network Link Conditioner will be available in System Preferences.
For versions older than Xcode 8, the package to be downloaded is called Hardware IO Tools for Xcode
. Get it from this page