Hello all, I am at wit's end. I have a MacBook Pro Retina 15.4 mid-2014 2.8 i7 with a NVIDIA GeForce GT 750M 2GB... I am trying to connect to an external Dell Ultrasharp U2515H. I have two cables - which one should I use? One is an Apple Lightning cable, the other one came in the box with the Dell monitor - one end is a mini DisplayPort, the other end is a Display Port. When I plug the cable in the Mac and connect to the Dell monitor, I get the message "There is no signal coming from your computer" - I have selected the correct input source on the monitor. Nothing happens. What am I missing here? Thanks!