I would like that have a motion sensor located at a point that way people walk by it, it triggers actors within Isadora.
The simplest way to do this: Video In Watcher -> Difference -> Calc Brightness
The Difference actor compares the current frame and the previous frame. Where ever they are different, you'll see grey/white areas.
By measuring the brightness of the frame using the Calc Brightness, you can discern if there is movement in the frame. The more movement there is, the higher the output of Calc Brightness.
The values coming out of Calc Brightness usually need to be scaled up because they can be quite small if you're looking at a large area and the amount of movement is not very great (e.g., a camera looking at people walking.) You can do this using a Limit Scale Value actor.
Finally you'll use a Comparator or Compare Guarded actor to trigger based on the output of Calc Brightness or the Limit Scale Value actor crossing some threshold.
See if that gets you going. ;-)