How can I connect Connect Nikon D5100 DSLR to my Macbook pro to be used as an external monitor?

I film videos and need a larger version of a view finder when dealing with my 50mm lens to make sure I'm in focus but I don't want to buy an external screen attachment.
My Mac is running MAC Os Sierra its a 2015 model.

I've been googling for weeks now and can't seem to find an answer. I'm open to buying an HDMI cord or whatevers needed but I can't find any information on the process.

This sort of feature is reserved for compact cameras that include a webcam mode. Unfortunately, DSLR cameras are not normally considered as suitable for use a webcam use.

Although the D5100 is an excellent camera, if you are serious about filming videos, you will probably do better with a camera that is primarily a video camera. Although modern still cameras (compact and DSLR) have video recording capabilities, for best results I have found that using a video camera for movies and a compact or DSLR for stills give the best results.

Sorry - not much help.

Fngovdn

Your Google-foo is weak, grasshopper.

includes videos as well.

What you seek is called "tethering" the camera to the computer.

"How can I connect Connect Nikon D5100 DSLR to my Macbook pro?"

The Apple article linked below does not show the D5100 as a supported camera, but it's worth a try…

Years ago, when you bought the camera, a USB cable was in the box. Connect the cable to the computer and the camera. Turn on the camera and select the movie recording mode.IMovie should open. If iMovie does not open, check to be sure the USB cable is well connected. Check to be sure the iMovie on-screen "switch" is in the camera position. Then, with Movie still open turn the camera off and then back on.

Have you ever heard of "Smart Shooter"-software?