Why aren’t my Safari App Extension NSLog messages showing up in the console in Xcode?

Switch to the scheme for your extension (schemes are just to the right of the Run and Stop buttons), and then hit Run.

A Popup will ask you to choose an app to run: select Safari.

A new instance of Safari should open and you'll start seeing log output in the Xcode console.

If you haven't signed your app and extension however, Safari will reject your extension and the console will show a message like

2017-04-12 13:00:44.799843-0400 Safari[37188:2787364] [Extensions] 
Computing the code signing dictionary failed for extension with 
identifier com.your.app.extension
2017-04-12 13:00:44.799865-0400 Safari[37188:2787364] [Extensions] 
Disabling and blocking extension with identifier: 

In this case you just need to re-check "Allow Unsigned Extensions" in Safari's Developer menu and enable the extension in the Preferences pane, after which you should be good to go.