I have been using older versions of Poser.  Just getting used to Poser 12 trial version, and probably will get Poser 11.3. My older Poser scenes load into Poser 12 OK, except the floor (ground).  I have been through loads of settings, but just can not see how to show a floor. Any help please.

Try shortcut Ctrl + G

But your description is a bit vague. Are you trying to show the ground plane? Are you trying to show a floor from a set?

Open hierarchy editor and see if ground is toggled visibility off or on

If you prefer to not use the enclosed "construct" and like the old style Poser Ground plane there should be a pre Poser 11 preset in the scenes library...

You can choose your startup scene.

An all-surrounding environment like the 'construct' gives better results for PBR Superfly uses. 

So: if you only use Firefly load/build your favourite start scene and set that scene as startup scene.

If you want to use Superfly without a background you can disable 'visible in camera' for the construct. It then has its effect in lighting but it is not shown in the camera.  

Try a different camera, your camera is probably under floor level... or your figure is somewhere, all the way up!!