Lifesteal through data editor doesn't work. You have to do it through triggers with the 'unit takes damage' event. For a simple formula that heals 10% of damage done per stack of the behavior, you can do it this way.
if
-'(any) unit takes damage'
conditions
-'unit (damaging unit) has behavior "Lifestealing."
variables
-'lifesteal' (integer) "Stack of behaviors "Lifestealing" on "Damaging Unit"
-'amt' (Real) "triggering damage amount"
-'Final' (Real) "No value"
Actions
Set variable 'Final' = (Arithmetic('amt' / '0.1'))
Set variable 'Final' = (Arithmetic(Final * (convert integer to real(Lifesteal))))
Set 'damaging unit' life to 'Arithmetic(damaging unit life (current))+'Final')
@Zantai: Go