You should be able to update pse 12 to camera raw 8.4 by using Help>Updates from within the pse 12 editor and then you should be able to open the nikon D3300 files in pse 12.
If your using windows xp or vista or mac os x 10.6.8, then you won't be able to update to camera raw 8.4 or use the 8.4 dng converter.