Have a look at one of the pork tenderloin threads -- same kind of question about marinade.
Basically, when you're ready to cook, you want to take the meat out of the marinade and dry it (the meat, that is) thoroughly with paper towels. Then you can grill or pan-fry or broil it (however you normally cook steak). When it's finished cooking, let it sit on a platter or cutting board for 5 minutes or so before you cut it. It's important to cut it in thin slices against the grain (across the short way) and on the diagonal from top to bottom (kind of like \ or / )-- otherwise the slices will be tough and all your care will go for nought.
As for the potatoes: I like cut the potatoes in chunks, toss them with just enough of the leftover marinade to coat them, and roast them in a baking pan in the oven. (Throw out any marinade you still have; you can't use it uncooked, and it will get ugly if you try to boil it.)