It's in an additional download. Use this menu item:

Xcode > Open Developer Tool > More Developer Tools...

and get "Hardware IO Tools for Xcode".

For Xcode 8+, get "Additional Tools for Xcode [version]".

Double-click on a .prefPane file to install. If you already have an older .prefPane installed, you'll need to remove it from /Library/PreferencePanes.

You can also install any of the Hardware IO Tools without installing XCode itself. Simply visit Apple's Download Center and search for "Hardware IO".

Update on the answer December 2019 Xcode 11.1.2

Apple has moved Network Link Conditioner Tool to additional tools for Xcode

Go to the below link

Install the dmg file, select hardware from installer

select Network Link conditioner prefpane

The tools can now be found ("Hardware IO tools") here:

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

1. Remove "Network Link Conditioner", open "System Preferences", press CTRL and click the "Network Link Conditioner" icon. Select "Remove".
2. Restart your computer
3. Download the dmg "Hardware IO tools" for your XCode version from , you need to be logged in to do this.
4. Open it and install "Network Link Conditioner"
5. Restart your computer one last time.