To assign Office365 License for mailbox or several mailboxes, login to the MSPControl and click on Mailboxes,
or go to Organizations > Exchange > Mailboxes.
You will see the list of all existing mailboxes. If you have many, it might be a good idea to use Search form in the right to find the one you need. You can choose whether you want to search by Display Name, E-mail Address, Domain Account, Account Number, or Login.
Once you found the mailbox(es) you were looking for, select them by checking the corresponding check-boxes in the left.
Then select Assign Office365 License in the drop-down menu Actions, and click Apply.
In the new window select a license from a list and click Assign.
Note: you can assign more than one license for a mailbox, and use one license for several mailboxes.