First off, I'm insanely jealous of your camera. What a marvelous device!
As of the most recent OS X Camera Raw Update, released on 5/13/16, the Nikon D5 is not supported. Unfortunately (and unbelievably), this makes iPhoto all but useless to you until OS X is updated to properly support the D5. There is not an announced ETA.
In order to make the most use of your camera on the iMac, you'll need pro-grade photo management software that has already been updated to support your device. What with the deprecation of Aperture, Adobe Lightroom may be your choice...and was updated with support for the Nikon D5 three months ago.
Lightroom is not free, unfortunately, but it's also not going to bankrupt you. A package including subscriptions to both Lightroom and Adobe Photoshop would run you about $10/month.
Apologies that I can't provide an alternative solution -- sometimes there just isn't another to be had -- but I hope this points you in the right direction!