Enabling Power BI Pro

I imagine this depends on just exactly how things were bought, but for me, I go to https://portal.office.com and logon with my administrator credentials. I go to Users, select the user I want and then expand licenses and assign them the license.

Anonymous

If you are the administrator, you are already enabled. If you need to assign license to other users within your organization, you can do it in O365 Admin Center. 

1. In the left navigation bar, select Users > Active Users.

2. Select their name > Edit.

3. On the user details page, select Licenses in the left navigation bar.

4. select Power BI (free), or Power BI Pro, then Save.

Reference:
Administering Power BI in your organization

Regards,