How to insert a (natural) symbol above a note using text eg D#... Db ...... D ? (natural)

Assumign this is plain text and not chord symbols, the shortcuts are Ctrl+Shift plus b for flat, # (or 3) for sharp, h for natural. You can also add them from the Special Characters palette by pressing F2 while editing text or clicking the button at the far left of the text toolbar. You can also add the symbols by themselves from the Symbols palette - press "Z" to display that.

If these are chord symbols, natural signs shouldn't really be used and will cause confusion when it comes time to possibly transpose them. But the same techniques do work, as long as you promise not to transpose them..