The tabletPressure is a property of the Path item. You'll need to create a path item first, then create the circles based on that path's position and pressure at each point

So there is no way of actually drawing the circles using freehand pen pressure?

Well, I don't think that scriptographer can't see a 'live' view of the pen's pressure like AI's brush tool can. But you can use the SG tool to trace out a series of very short paths, and run the bubbling script on each piece in turn. That would give a very small delay.