My experience with PowerBI is that in Office 365 app launcher you will see a yellow tile that takes you to app.powerbi.com.
I have enabled the Power BI license for other users and they are seeing a blue icon that takes them to sites.powerbi.com and they only see a blank page.
this it the same URL that you get with the old SharePoint PowerBI app
in the O365 tenant > license I have enabled "Microsoft Power BI for Office 365 Add-on" which I believe is the Power BI Pro license.
am i missing something?