Can't drag and drop UI elements from Storyboard to Assistent Editor (into the code) - Xcode 7.3

I am following the instructions of https://developer.apple.com/library/ios/referencelibrary/GettingStarted/DevelopiOSAppsSwift/Lesson3.html#//apple_ref/doc/uid/TP40015214-CH22-SW1

I can't drag and drop the UITextField, UIButton and UILabel to the ViewController code. It just won't drop.

What am I missing to do?

I obviously did not change the name of the view controller and I am just following the steps of the documentation.

Are there any options that I need to check in order to be able to do this?

Thanks in advance.