Instead of core.jar, I imported the processing-core.jar. I don't know what's the difference, but the second one seems to work for me (maybe core.jar is okay too, but I can't remember).

If you can't find the processing-core.jar, do this:
1. start Processing (not eclipse) and write anything
2. turn on the Android mode (first you have to save it, i saved it on desktop) and then export the sketch
3. open Eclipse, start a new android project and when you have to import the processing-core.jar go to the exported sketch C:\ Desktop\ SKETCH\ android\ libs and in the libs directory you'll find the processing-core.jar

I suggest you to copy the processing-core.jar to a safer location so you can find it anytime you want.

In your project, open the src>your.package.com (your package) and write your code, here's an example:

1. package patak.proba.net;
2. import processing.core.*;
3. public class ProbaActivity extends PApplet {
4.     public void setup() {
5.         size(480, 800);
6.         background(255);
7.         smooth();
8.         fill(0);
9.         stroke(0);
10.     }
11.     float x, y;
12.     public void draw() {
13.         background(255);
            x = width/2 + 50*cos(  (float)(0.0032*millis())  );
            y = height/2 + 50*sin(  (float)(0.005*millis())  );
14.         ellipse(x, y, 30, 30);
15.         line(0,0, x, y);
16.         line(width, 0, x, y);
17.         line(0, height, x, y);
18.         line(width, height, x, y);
19.     }
20.     public int sketchWidth() {
21.         return 480;
22.     }
23.     public int sketchHeight() {
24.         return 800;
25.     }
26. }

Thanks for the post.  I am going to try the solution you posted tonight and see if it works.  I've been stuck on this for at least a week..