Instructions
Create a character with "Nature" or "Sorcery" powers. This is chosen during the "Select Power" portion of creating your character. Only these two power sets can heal.
Level your character to level 10. This will unlock the "Healer Role." Before level 10, you only have access to the default "Damage" role.
Press "Up" on the PlayStation 3 controller's D-Pad to bring up the "Actions Quick Menu." Select "Switch Role" and press "X" to switch between "Damage" and "Healer" roles.