Hi, I'm Ashik. Welcome to JustAnswer. I'm reviewing your question now, and will post back with your reply ASAP
Old Nikon application will no longer install on latest El Capitan due to some compatibility issues. You can download and install the latest Nikon Capture NX-D from here --> http://download.nikonimglib.com/archive2/bjc3400voHTs02e2ijh45ni9h004/S-NXD___-010401MF-ALLIN-ALL___.dmg
Latest Nikon ViewNX-i can be downloaded from here --> http://download.nikonimglib.com/archive2/FEUzE00e0LZ202b7uuF07gjnrQ05/S-VNXI__-010100MF-ALLIN-ALL___.dmg
You have to install both downloads.
(If you don't seethe downloaded Nikon Capture and View dmg files, then click on the Finder icon on dock, then click on Go menu and select Downloads. This will bring Downloads folder. Now you see a file named "S-NXD___-010401MF-ALLIN-ALL___.dmg" and "S-VNXI__-010100MF-ALLIN-ALL___.dmg" files there. Double click on it to open and then install them)
Note : Photos is the latest application which is a replacement of iPhoto. You can close Photos if it comes up when you connect your camera.
If you have any difficulty on following the steps above, please let me know and I will continue to help you