Usually you use the access code that your textbook has inside. Now, if you got that textbook USED, chances are that access code has expired and you will have to purchase a new one. But most of the time the access code on the textbook works because it proves that you have paid full price for the textbook and are capable of using the other services.
I had a similar experience. For my Economics class, we had to use the online website and I ended up buying the access code and just printing out the chapters off the website...usually places like mycomplab.com will provide an e-book which is basically your whole textbook on the computer. So look into that...find out if you can spend like $70 on the access code and the e-book instead of buying the text book for a more expensive price. It saved me money and I liked being able to print out my chapters as I went along and highlighting them!
Also, if your access code doesn't end up working, call one of the help lines they provide on the website and I'm sure you'll figure it out.