Ok lets make this short and simple.

Put the Planet#.pof in the data/models folder
Put the Planet##.pcx in the data/maps folder

If you are using software render, then you will need to use the 
Planet##lo-res.pcx, simply extract it to the maps folder in place of
the normal pcx, and delete the 'lo-res' bit in the filename.

Please note that these models are extreeeeeemely massive, so it is
best to place them very far away in FRED from the centre of the 
mission's action (but not too far away as FS will not normally render
something that is extremly far away from you, and this will cause
parts of the planets to appear to be cut away or not be visble at all)

The planets are intended to be used for aesthetic purposes, to add
something extra to your missions and campaigns, anyone who wishes to
use the planets in a more complex ways can feel free to do so,
but remember pilot, your on your own :)

Feel free to use these models in your own missions/campaigns as long
as I am credited with their creation, I got the texture maps from 
another site with a lot of real planet maps.

Oh well so much for short and simple.

My E-mail - robert@wild666.freeserve.co.uk