First of all, when you're going to use those sub or super position commands, you have to select the text and then press the appropriate button. If you press the button before or after you've typed your stuff, it won't do anything useful. The sup and sub tags have to be before and after the text you're targeting.
Percent yield is simply the experimental yield divided by the theoretical yield and multiplied by 100%. So, you need two kinds of information, some kind of information about the reactants and an "actual" measurement of the final product to compare the two.