How do i get the total frames and how do i get what frame is currently displayed?

I'm playing a bit with the terrain example (found on wiki) using a (heightmap) image. I added a cube and simple collisiondetection. It all works ok. Now i wanted to change the height of the terrain in realtime and i added a GenericVertexController, wich also works well. Now i like to change the height of the terrain only arround the location of the cube. The height of the terrain is stored in a float array terrain[][] and the height of the terrain in the GenericVertexController apply method is stored in a Mesh (SimpleVector[].y).

This is what i do now in my apply method:
SimpleVector[] d = getDestinationMesh();
int tx = (int) (Cube.getTranslation().x + 640) / 10;  //The + 640, terrain starts at -640 ends at 640
int tz = (int) (Cube.getTranslation().z + 640) / 10;  //In reality it was -560 to 560.. very strange also
int i = 0;
terrain[tx][tz]=-256;
for (int x = 0; x < X_SIZE ; x++) {
for (int z = 0; z < Z_SIZE ; z++) {
d[i].y = terrain[x][z];
i++;
}
}
this.updateMesh();

The above code does change the height on a certain location on the terrain, but not on the location of the cube. And i tried all kind of things. I must be doing something wrong..