If you are on Mac after the install the DNG Converter is now available in your Applications directory
On Win the DNG Converter is now available in your Programs directory, and from the Start menu, under Program Files > Adobe > Adobe DNG Converter or All Programs > Adobe > Adobe DNG Converter
Double click on the DNG converter and follow the instructions, the converted DNG files can then be imported into Lightroom
After installing the DNG Converter you can run/open it and use it to convert a FOLDER of raw (NEF) images to DNGs and can import those DNGs into an older LR.
There are usually one or two things that cause issues for people who are new to the DNG Converter and/or have a new Nikon camera:
1) The DNG Converter works on a FOLDER not an individually-specified image, so when you are telling it where the folder it, you should single-click on the folder to select it, not double-click to open the folder. Every other piece of software, including ACR, will allow selecting a folder by opening it but the DNGC won’t, you have to just single-click to highlight the folder name.
2) If you have touched (transferred, viewed) your new Nikon camera images with any older Nikon software (Nikon Transfer, View NX), like came with your previous camera, then the images are corrupted by using that obsolete software so Adobe says they are an invalid format. Try copying the NEFs with your OS tools, like Finder or Explorer, and then see if they were in the DNG Converter. If it is important to uncorrupt the few images you’ve taken, then you can use a utility from here:
FYI lightroom 5.7 now available and imports d750 files