To add to someones entitlement is a 2 step process. 

Firstly you want to update their details so that future leave years are added with the correct number of days (which is one in People > employee name > Person (dropdown) > Work and change his Base leave Entitlement) and Secondly you should update their current and future leave years with any extra days (Which is in People > employee name > Leave (dropdown) > leave Years)