You title leave room for misunderstanding. We better make fully sure, that we are talking about Target Display Mode https://support.apple.com/en-us/HT204592
Because that is, what the shortcut is fore. And that you have connected a displayport-source as input in the mini-displayport.
Ya I was using a Thunderbold cable but just ordered a Minidisplay cable because I read that somewhere....but it wont even enter target mode until it gets a signal or something? That must be it. Thanks for the tip. =)
Yes. I just dobbelt checked. Without video-input, there is no reaction. But it works with my Macbook Air connected.. If you didn't already have the thunderbolt cable a mini-display cable is also much cheaper ;-)