How to get screen x & y position of object on a 3D space?

Hello I´m working on an interactive 3d flock.

The idea is to be able to grab and click over the 3d boids. For that I don´t know how to get the x and y position of object that have x, y and z position…

Any idea?

The code is here on my blogs: http://www.patriciogonzalezvivo.com/blog/?p=170