I'm building an ability that drains energy from the target unit and gives it to caster unit. I've tried to associate an actor that hooks in to the ability's start event to create the continuous beam, but the beam never appears. What's the right way to do this?
Check out "DominationBeam" under actors. I think this is an ability that a campaign character could use that had a beam going from her to the target. You'll need to have events creating the actor when the effect of the ability starts and destroying the actor when the effect ends. Don't use the ability's start/end in the events, just the effect that is triggered by the ability.
The Host Impact fields are the ones where you will probably want it to use the scope "Scope Target" and the subject as "_Unit". Same with the Host Launch, except the scope is the caster. In the impact/launch attachment fields you'll need to put what points the beams should attach to.