Can't add Unit Tests to existing iOS app

I a trying to add Unit Test to an already working iOS App. I've searched and read many threads that address this problem, and followed their advices step-by-step but I seem to get stuck at a different place.

The app is an ordinary iPhone Application, that I am currently developing using xCode 6.3.1 I've been able to create the 'Cocoa Touch Testing Bundle' fine for my Application. A basic test gets created, but when I press the 'Test' button, I get the message 'The scheme "Project Tests" is not configured for testing. Edit the scheme to enable testing, or cancel the action.'

I press the 'Edit Scheme' button and go to the Edit Scheme Editor. But in that window (you can see a snapshot below this explanation) I am never able to choose any targets, all the options are actually greyed out! I've tried everything but I am not really sure what I should change in my project in order to change this and successfully create the Unit Tests... Any ideas please?