hi,
as long as you buy it from a reputable book store or online retailer (like amazon), then you should have no problem getting the 6 free tests. the instructions on how to access the tests come with the book. basically as was posted above, you have to make a login on their website, and they ask you to enter in a specific word on a specific page # to verify you actually own the book. then you enter in a registration code. after that, you have access to all 6 tests.
if you buy the book used, and if the previous owner already registered the book online, then i dont think it will let you re-register the book. in this case , you'd have to ask the previous owner for his/her login information so you can access the tests.
in case you're wondering, you can take each of the 6 tests one at a time, but you can't retake any one of them until you finish all 6. so for example, if you've taken 4 of the 6 free tests, you will have access to the score reports and explanations for the 4, but you can't retake any of those 4 until you finish the last 2. once you finish all 6, all the tests will reset (i believe), so you wont have access to the test reports, etc. for any of them (they will all be blank).