on this one i think i got a bit lucky - there was just a minor bit of crustiness in a couple of places when fully zoomed. one trick i do know is to decrease the Raystep multiplier and Stepwidth limiter on the Calculation tab. looks like they default to 0.25 and 0.3 in high mode, respectively. i'll drop them down to 0.05 and 0.1 respectively if i'm seeing a lot of spotting. increases your render time by quite a bit, but it makes a big difference. hope that helps.
it's an iterated render system, kind of like you see in apophysis or chaotica, where it progressively renders the image. so the first run through is a bit noisy, but after each iteration (run through) it progressively cleans up the image. i don't understand it much beyond that, but there's lots of neat options in it, too.
you can access it by clicking on the Utilities tab on the main window, and clicking the M.C. button. then you import your parameters (by clicking the Import parameter button) and select your options, and click Start rendering. and as i said, it will take a fair bit longer, so if you have a slow machine like me you probably want to let it render overnight. whenever it looks good to you (or the image stops getting better looking) you can hit Stop rendering and then save.
sorry if that's a bit vague - i'm still somewhat new to mb3d myself, and i don't understand M.C. rendering much beyond what i've just told you.