Oops, yes, I forgot to answer your original question.
You need to create a Proc object and pass it as an argument. The Proc can be anything you wish really, you can have it call a method, change a variable, etc.
I do not know what you want to do, so its a bit hard to tell you what argument to pass, but here would be an example:
Lets say that I want to flip a switch whenever the pathfind successfully completes:
Pathfind.new(
Node.new(8, 6), 13, 0,
Proc.new { $game_switches[1] = true })
The node is the coordinates, 13 is the event ID, 0 is the range, and the Proc is the procedure that will be executed on a successful route completion.
I am not aware of a pathfinding script that supports 8-directional movement, but to be honest I have never really looked into any others, so that does not mean to much.