Elements 11 is only capable of using up to Camera Raw 7.4. You will either have to upgrade to Photoshop Elements 13 or download and use the latest DNG converter to create DNG copies that you can use with what you have now.
Thanks Jim - I thought that may be the case, I'll upgrade to 13 over the weekend
You ought to be aware that after you install Elements 13 it will be necessary for you to run an update from the help menu in order to get Camera Raw updated. Otherwise, even installing Elements 13 won't open the raw images from your D750.