Use the <sup></sup> HTML tags. Below the message editing window, you should have an HTML view checkbox (if you are using Internet Explorer; Netscape gives the HTML view by default, I think). Here's some examples:
SO4<sup>2-</sup> gives SO42-
Na<sup>+</sup> gives Na+
It's a little extra work, but I use font size 3 for superscripted minus signs to make them bigger so they're easier to see.
You can also use the <sub></sub> tags for subscripts, but I usually just choose font size 1 to make small subscripts since that's easier. Just highlight what you want to subscript and choose font size 1. I use font size 2 (Arial font) for my normal text. I've found this font to work most reliably with different sizes associated with subscripts and superscripts.
Steve