In the editor, you must click on the "advanced" tab, and then on "gameplay constants". In there, you will see a field "Hero XP gained - Creep reduction table". This tells you how much experience a hero will gain from creeps on which level. By default it is "80, 70, 60, 50, 0", so this means that your hero will gain 80% of experience from creeps when he is level 1, 70% when level 2, 60% when level 3, 50% when level 4 and 0% when at level 5. Because this is the end of the list, your hero wil also gain 0% when his level is higher than lvl 5. Change this field, if you put in only 100 then all heroes on your map will gain 100% experience from creeps no matter their level. | 
