In order to take the Splunk Admin certification exam, you must first complete the following courses:
Splunk Fundamentals 1
Splunk Fundamentals 2
Splunk System Administration
Splunk Data Administration
As you have already completed Power User certification, you have presumably passed the first two courses. If you have not yet passed System Administration and Data Administration, you'll need to register for those through Splunk Education, but they are not free courses. Once you have passed those courses, you can contact Splunk Certification to register for the free exam.