Depending which Mac keyboard you have, try function key, fn, and hold down followed by Option key+N, this seems to work on the wireless keyboards (compact and full size).
TBH it's a bit of a stretch on the full size keyboard to hit fn+option+n and I find it much easier and faster just to right mouse click anywhere in the image then select from the context menu 'Mode' > 'New Preview' or click on the 'New Preview' icon in the top toolbar.