This short blog shows you how to assign Power BI Pro license to user after you purchased them.
First you will receive an email like this:
Then you will get to the Business Center.
Select an account to manage.
You should see Power BI Pro in your product list.
Select Manage services.
You will brought to the Office 365 Admin Center.
From here you can assign the license.
If you cannot get to the Admin Center then there is no way for you to assign the license. You need to either look for your vendor for help, like me, or there is a way for you to contact Microsoft support, which I could not get to do that. The support number that you can find in Business Center is completely useless, that guy is going to tell you “I only help you to activate the license, the rest you find your own way”.