How to upload photos from my Nikon D5200 to my mac?

I recently updated to El Capitan and IPhoto will no longer work.

IPhoto was replaced by "Photos" a while ago. If that upgrade has not already happened, that is step 1.

Another issue may be in preferences. In this case, connect the camera's USB port to the Mac's USB port with a USB cable and get the camera into PC or computer mode. In "Applications", launch Image Capture. Set the default for Photos (or iPhoto) to open when the camera is connected.

If none of this works, then get an external card reader (if you Mac does not have one built-in). Take the card out of the camera and insert it to the car reader. Launch iPhoto or Photo (if not automatically launched) and import the images directly from the card.