I've tried to import NEF files into Lightroom (Nikon 1 J5) from my iphoto library, without success. I've also tried to convert them with the DNG converter, but always get the error message 'The source folder does not contain any supported camera raw files.' I'm not sure what to do at this point to even get started with importing the photos so would be grateful for any advice!
The photos in iPhoto cannot be directly imported into Lightroom. You have to export the photos from iPhoto first. As I have never done this, and don't use a Mac, I advise you to search the forum or the internet for complete instructions.
What version of Lightroom are you using? Support for the J5 was added in LR 6.1/2015.1. If your Lightroom is older it won't import the files. If you have LR 6.0 installed you should update your installation. If your Lightroom is v5 or older, you can indeed use dng converter. However, your DNG converter also needs to be new enough to support those files if you have an older version of Lightroom. It is a free direct download from Adobe. You can indeed directly import from iPhoto if you open up the package or do a "show in Finder" which is probably what you did.