The language the forum uses is LaTeX. There is a guide somewhere on the forum for how to do it. You can also click other posters equations to see the TeX code that generated it, that's a good way to learn. LaTeX is heavily used in scientific writing so a simple googling will find a treasure trove of info.
Not that I know of outside of going a step further and downloading a compiler. If all you want is to type a few equations on the forum, I think just writing it up and posting it is the best option. Then you can just edit it until it's right I suppose...