To assign the available resource licences (worker, work equipment or vehicle), go to the My Resources -> My Licences tab and click on the type of licence that you want to assign. For example, if you want to assign a worker licence, click on Number of worker licences:


Then you must click on the icon 

to assign the licence to the worker you want:

The licences are non-transferable, so once you assign them, they cannot be assigned to another resource.