Hi ankban,
Permutations, Combinations and Probability are all relatively rare categories on the GMAT - you will likely see just 1 of each on Test Day (although you might see more if you are performing at a particularly high level in the Quant section AND the 'randomizer' gives you some additional tougher questions in these categories - although you could just as easily see upper-level Algebra, Geometry, Rates, etc. in this situation). Permutations and Combinations are based on the same general math ideas (although you ultimately have to determine whether the "order" of the events in the question matters or not - so that you can make sure that you're correctly doing the math that's required. Probability is often about working through a step-by-step multiplication of fractions. Thankfully, once you learn the basic ideas behind these questions, the work needed to solve them isn't typically too tough, but you have to stay really organized and do a lot of proper note-taking.
Before I can offer you any additional advice on these areas, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied in total? How many hours do you typically study each week?
2) What study materials have you used so far?
3) On what dates did you take EACH of your CATs/mocks and how did you score on EACH (including the Quant and Verbal Scaled Scores for EACH)?
Goals:
4) What is your overall goal score?
5) When are you planning to take the GMAT?
6) When are you planning to apply to Business School and what Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich