*sigh* This one is a pain in the ass. Like the more successful real-world diseases, it's constantly mutating and changing, to avoid killfiles as well as the ever-more alert populace. It relies on a constant human impulse, greed. But it is always the same at heart, and a little thought before you follow its instructions will quickly show you what's wrong.
The basic scam is this: the letter contains five addresses. It says "Send a dollar (or two, or three) to each of the names on the list. Then take the top name off the list, put your name at the bottom of the list, and send this letter to a bunch of other people. Pretty soon they'll all be sending you money, and you'll be rich!"
Ok, come on. First off all, despite anything that the letter says to the contrary, this is completely 100% illegal, at least in the United States, and probably elsewhere. It is illegal because everybody but the last people that "play" make money.. and those last people get shafted.
Here's a little example to help you think about it. Imagine there's eight people in the world. Each one has two dollars. One person starts. She writes up a letter, and puts a list of two names, both hers (at least, that's what I'd do if I was running a scam like this), and sends it to two other people. They each send a dollar to each of the names on the list, and send two copies of the letter to other people. At the end of the round, then, person A has her original two dollars, plus the two dollars she got from persons B and C, making a total of six. B and C are now both broke, and D, E, F, and G all have their original two. Next round, D and E receive the letter from B. They send one dollar to A, and one to B. F and G receive the letter from C, and send one dollar to A, and one to C. Now look. Person A has the six dollars from before, plus one dollar each from D, E, F, and G, for a total of ten. B and C each get two dollars, so they're back to their original investment. D, E, F, and G are all broke. Now what happens? Probably one of D, E, F, or G sends the letter to H, the only person who hasn't played yet. But even then they'll only get one dollar from him. So everybody else who can't pass the letter on anymore just gets screwed. And A, with ten dollars, laughs all the way to the bank. Thanks, guys.
The only way to make money from MAKE.MONEY.FAST is to be one of the original starting people. And even then, you assume that you won't get caught (and you are handing out your address to people so they can send you money), and you're assuming people won't wise it. It's possible to make money this way, sure. But your chances are probably better with a slot machine.