Use the event,
Generic Unit Event - Unit -A unit is attacked
The conditions
Integer Comparison - Level of Rejuvenation for (Attacked Unit) is greater than 0 [This checks if the unit being attacked has the Rejuvenation spell, since having an ability at level zero is not having the ability at all, anything higher means that they do have the ability]
Real Comparison - Life of (Attacked Unit) is Less than or equal to (YOUR VALUE)
Actions
Unit - Order (Attacked unit) to Night Elf Druid Of The Claw - Rejuvenation (Attacked Unit)
This only works when a unit is attacked though, so you would need a Damage Detection System, but don't worry about that for now. Sometimes there is a little hint on top of the Ok and Cancel box, when you are making triggers, it's good to read them when starting out. Try Googling what each Event is for and when is it appropriate to use Matching unit, Picked Unit, Attacked Unit, etc.