The title of this section is a bit misleading. In order to administer Power BI, you have to go to Power BI. There is no admin center in Office 365 admin centers for Power BI:
Once you go to Power BI, you can click on the gear icon and choose Admin portal from the drop-down menu:
The following is the home page for the Power BI Admin portal, which shows usage metrics for Power BI. There are also a few additional sections in the portal. Let's go through them one by one: