If you are not going to buy Fable 2 then this game is not for you.
To begin, you must choose your avatar, but since you don't have a profile yet, you have to choose one of the preselected characters (don't worry, you can merge him/her with yours later). Then, you simply choose the game you want.
Fortune's Tower is the most entertaining of the 3 games. You place your bet at the start and the dealer lays down a row of cards (one facing down and two facing up). There will be a number displayed to the right on how much the row is worth and you win more depending on how much money you bet. You have the choice of taking the deal or to keep going. If you choose to take the deal, you win the money and the round ends. If you choose to keep going, the dealer lays down the next row of cards. This continues for seven rows. However, you don't want to get a pair vertically. If this happens, you lose, but there are two ways to get out of it. One way is if you get the hero card which is worth nothing but it cancels out all pairs in the row. The other way is your gate card (the first card that was faced down) will automatically be used to cover up the pair. Unfortunately, this doesn't always save you. Once you understand how to play, it can be quite addicting and it's always best to know when to stop.
Keystone is a dice rolling game. First, you take bets on the stones that are right next to the numbers, 3-18, these bets last the whole game. Then, you place your inside bets where you bet on numbers again as well as if the number rolled will be odd or even, be black or red (some numbers are colored), etc. Once your done, the dice is rolled and you win money for every bet that you got right. This continues until 2 of the "keystones" (3, 18, 10, and 11) have been rolled. You lose the money that you bet on the stones if they're not rolled and a new round starts. This one can win you a ton of money but it's a lot of luck. There's even a game called Bloodstone but you bet on what you don't think will be rolled. Sounds easy but you don't win that much with your bets and you lose a lot if the number is rolled.
Spinnerbox is the worst of the 3 games as it is nothing but 100% luck. It's pretty much just a slot machine and the chances of you winning much money are slim. You might want to just do yourself a favor and avoid this one, especially if you're trying to win money for your Fable 2 profile.
On top of playing these games, you can also enter tournaments and not only win more money, you can win items to. There are 2 rounds in the tournament. In the first round, you have to place in first in order to continue. In the second round, you get money just for placing (a lot more if you win) and you win the item no matter what.
You also have a "Gamblers rating" which is a level system which can unlock certain games and tournaments. You level up by simply playing. You don't have to win, just play.
So, if you plan on buying Fable 2, it might be worth a shot BUT only if you preorder Fable 2 and get these games for free as it's probably not worth the 800 mp ($10). If you don't plan on getting Fable 2 at all, then don't bother.