metaballs by point alternative

Hi all,

i'm a student for architecture in Florence. In my last project i want use the metaballs but i have a problem to scalar every single metaball respect to a point as seen in this video

http://vimeo.com/29765097

 
How to make a point attractor to use whit the metaballs??? I made several try by entering in the input "t - isocurve treshold value" the distance between the points of the generating metaballs and the point attractor but does not work...

help me :)

p.s.:sorry for my english