Xcode does not highlight the selected instance in the editor.
Xcode - Text Editing preference has an option to enable/disable selected code/symbol highlight. And it's enabled there, but it still doesn't work in the editor.
Here is a snapshot of my editor code, with the selected instance.
I'm using Xcode 9.3-beta2.
Here is a similar question, but it's related to changing the highlight color: Make Xcode highlight selected symbol more
Did you try just to click on a symbol? If I do that it will highlight all instances. Below I just placed the cursor on NSNotificationCenter
by clicking on it. So, do not select the word, only place the cursor in it.