So for my first WordPress MultiSite Project i studied that function and found out that I have to edit the wp-config.php, which I did like everyone describes.
Unfortunately, even after multiple attempts, it still didn’t show the “My Sites” on the top menu bar of WordPress Backend Dashboard.
So I decided to try few different things, but even a complete clean-up of the database and reactivation of that function didn’t help.
So I found this WordPress MultiSite activator Plugin
… And it did the trick for me.
It reactivated that My Sites Menu in WordPress Dashboard just like it should “naturally” happen.
Here we go.
So instead of reinstalling whole WordPress, try this solution first and let me know in the comments if it helped you too
While writing this article and having the problem I am on WordPress 4.2 by the way.