If you have issues accessing a variable that you expect is likely a variable (for example an id number due to it often being a unique value separating one object of the same type to another), it's probably because that variable is not publicly accessible. Looking at the code, this is the case here.
You need to make @actor_id publicly accessible. A quick addition to the scripts:
class Game_Actor
attr_reader :actor_id
end
will do the trick. Accessing it can be done via actor.actor_id
If you wanted to change it to be simply actor.id, you can add this code instead:
class Game_Actor
def id
return @actor_id
end
end
These IDs are the same IDs as in the database. So actor 1, by default, is Ralph, actor 2 is Ulrika etc.