I have just taken mass & balance and I tried to learn all the definitions, standard mass values for passengers, baggage etc along with all the equations you need for %MAC, moving CG, moving mass, adding mass etc. If you know all these you should be fine.
Some of the questions that I used to practice gave standard mass values and some didn't but the questions I had in the exam gave them (assume 1 passenger = 75kg for example).
The only questions I had on definitions were about what on a short flight, the traffic load is limited by.. questions like that.
For mass & balance I found that just practice, practice, practice and it was all good. Pretty much all the questions I had seen before, just the figures had been changed. Got 97%.