Hey Erick Chan,
Thanks for posting.
As I remember, you do not already have FileMaker Server and are just testing correct? You will need to download FileMaker Server trial. During the install of FileMaker Server trial, you will install FileMaker API for PHP.
To download the trial, you can follow the link that you posted and there will be a link under the How do I get it? section.
Let me know if you have any further questions.