Try this
FrameLabel -> {Style["[Delta]", 13], Row[{Style[Subscript["N", "P"], Italic], " g", Superscript[m, -2]}]}
Following Gianluca Gorni's advice is what you want to do. That will show you all sorts of structures besides the superscript and subscript. The keyboard equivalent for getting just the two structures you're currently using follows:
Have you tried the basic math input palette?
Sorry .. but How helpful is that..? i have my own data here..
