The button that you clicked to begin the first exam is the same button that you click to continue taking additional exams - it's a "start exam" button in general, not a "take the 1st exam only" link. You can keep clicking on that same button to start new exams.
This assumes that you started your account correctly. Anyone can start an account and get one free exam, but if you bought the books, then you have to type in a special code in the book in order to get access to the remaining exams. If you don't do that, or if you don't do it correctly, then the system won't give you access to all of the exams.
If you didn't put the code in yet, go do it now. If you did but are still having problems, or if you need any help, please contact the office (800.576.GMAT) and they will walk you through what you need to do over the phone. (Alteratively, if you're not in the US or Canada, you can send an email to
studentservices@manhattangmat.com with your phone number and they'll call you back.)
Note to others reading this: in general, when you need technical or account help, contact the office at 800.576.GMAT or
studentservices@manhattangmat.com - the forums are not the best place to get these issues resolved. (We're just teachers answering questions about the exam here - we don't know how to fix any technical problems. :)