To view the app registrations follow the following path: -
Goto Services and find App Registrations
Click on App Registrations and set the filter to All Apps
Then search of the name of the application as used when the app was registered on the PowerBI portal (in this case 365-hcpm-demo Power BI Integration )
Once you click on the app you will be able to see the details
At this location one should be able to view and manage most of the properties of the application.
Note: The secret key is only visible once when it is being created and thereafter hidden so if at all you have forgotten your key please delete the existing one and create a new one.
Name, Object id and other properties are visible using the properties tab page.
Reply urls tab page is used to alter the redirection page.