If you can type an answer you are almost there. WHen you put text in the box do you see numbers underneath showing how many characters you have typed out of the maximum 4000 allowed (e.g. mine now says 199/4000).
If you click 1 centimetre t teh right of those numbers that's where the submit button is. Try clicking there- try it on this thread
If it is there but greyed out, it can be if you pasted the text into the box rather than wrote it in. Should that be the case then try hitting the greyed out button anyway, or edit your post then check again.