Alice, I haven't tried the magnetic placement mat yet but I am going to try a magnet sheet today that I found in my scrap drawer. But with the BS/BK, you can use the crease pad and that will cut and emboss in one pass. I usually run it twice though, I turn the handle so the plate rolls away from me, then I roll it back towards me, and I get a nice emboss with that.
You use tab 2 on the Multi Purpose Platform (MPP), cutting pad, die blade up, cardstock face down, crease pad, roll through machine.
I can see how the magnetic mat would be a big help to keep the dies still though. I've heard good things about them.
Alicia over in the Big Shot FAQ thread is awesome about answering all my questions (even the strange ones that I seem to have, LOL), so if you need more help, ask here or ask over there and I'm sure someone here on SCS will know the answer. HTH and Happy New Year!