New camera model = new file format = need new software to read the files.
The D750 requires LR 5.7 or newer for native support.
Alternatively, you can download the latest DNG converter, convert the D750 NEF files to DNG then import the DNG files.
The Nikon D 750 requires camera raw 8.7 which is not compatible with Photoshop elements 12 and Lightroom 4
You can use DNG converter to convert your files
DNG Converter 8.7
So... It sounds like I am going to have to pay to upgrade my software... Ugh...