I have purchased new camera, a Nikon D750, and now
my Photoshop version CS5.1 with the cameraw raw
plug-in version 6.7 doesn't support the files from it.
Which version of the raw converter plug-in can I install
to tackel this problem?
That camera requires Camera Raw 8.7 or later (or Lightroom 5.7). The last version of Camera Raw supported in CS5 was 6.7. So Richard is correct. You will either need to update to CS6 or newer in order to run a more recent version of the ACR plugin, or you will need to use the free DNG Converter utility to convert your files to DNG before importing.
You can find the latest DNG Converter downloads here: