My favourite way of inserting properly typeset formulas into plots is to compose them first in a Text cell, within an inline cell. Then I copy the inline cell and paste it into the input within quotation "". What I get, behind the scenes, is something like this:

Plot[x^2, {x, -1, 1}, Frame -> True, 
 FrameLabel -> {"\!\(\*FormBox[\(\*SubscriptBox[\(N\), \(p\)] \
g/\*SuperscriptBox[\(m\), \(2\)]\),
TraditionalForm]\)"}, BaseStyle -> {FontFamily -> "Times"}]

except that I see the readable typeset version.

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:

Sorry .. but How helpful is that..? i have my own data here..