it's a bit vague since you don't describe your scene / show code

but do you know
http://www.processing.org/reference/screenX_.html

Ok I'll try to explain a little more.

You could turn it on it's head and put a 'cursor' in the 3D scene that reacts to mouse movements. For example draw an empty rectangle (around a chess-board square for example) that moves within certain limits (the board size) depending upon mouse movement.

yeah, you could map the mouse-y-value onto your z-axis

1. float CameraX;
2. float CameraY;
3. void setup()
4. {
5.   size( 800, 800, P3D );
6.   CameraX = width / 2.0;
7.   CameraY = 50;
8.   noCursor();
9. } // setup
10. void draw()
11. {
12.   background(0);
13.   stroke(111, 110, 110);
14.   fill(111, 110, 110);
15.   // ==================================================
16.   // draw floor
17.   float floorY=600;
18.   line ( 20, floorY, -200,
19.   width-20, floorY, -200);
20.   line ( 20, floorY, -20,
21.   width-20, floorY, -20);
22.   line ( 20, floorY, -200,
23.   20, floorY, -20);
24.   line ( width-20, floorY, -200,
25.   width-20, floorY, -20);
26.   // ==================================================
27.   //float floorY=600;
28.   line ( 20, floorY, -300,
29.   width-20, floorY, -300);
30.   line ( 20, floorY, -20,
31.   width-20, floorY, -20);
32.   line ( 20, floorY, -300,
33.   20, floorY, -20);
34.   line ( width-20, floorY, -300,
35.   width-20, floorY, -20);
36.   // ================================================== 
37.   // camera
38.   camera(
39.   CameraX, CameraY, 700,
40.   width/2.0, height/2.0, 0,
41.   0, 1, 0);
42.   // ==================================================
43.   // paint boxes
44.   //MyBox ( 310, 20, 22, 530, 210, 3, 14, color (255, 0, 0));
45.   //MyBox ( 110, 440, 44, 52, 10, 222, 14, color (2, 0, 255));
46. 
    
47.   MyBox ( mouseX, 590, map (mouseY, 0, height, -610, 110),
48.   mouseX, 596, map (mouseY, 0, height, -610, 110),
49.   14, color (2, 0, 255));
50. 
    
51. } // draw
52. 
    
53. void keyPressed() {
54.   if (key == CODED) {
55.     if (keyCode == UP) {
56.       CameraY-=10;
57.     }
58.     else if (keyCode == DOWN) {
59.       CameraY+=10;
60.     }
61.     else if (keyCode == RIGHT) {
62.       CameraX+=10;
63.     }
64.     else if (keyCode == LEFT) {
65.       CameraX-=10;
66.     }
67.     else {
68.       // nothing
69.     }
70.   }
71.   else { 
72.     // not key == CODED
73.     //
74.   }
75. }
76. void MyBox(float x1, float y1, float z1, float x2, float y2, float z2, float weight, color strokeColour)
77. // was called drawLine; programmed by James Carruthers
78. // see http://processing.org/discourse/yabb2/YaBB.pl?num=1262458611/0#9
79. {
80.   PVector p1 = new PVector(x1, y1, z1);
81.   PVector p2 = new PVector(x2, y2, z2);
82.   PVector v1 = new PVector(x2-x1, y2-y1, z2-z1);
83.   float rho = sqrt(pow(v1.x, 2)+pow(v1.y, 2)+pow(v1.z, 2));
84.   float phi = acos(v1.z/rho);
85.   float the = atan2(v1.y, v1.x);
86.   v1.mult(0.5);
87.   pushMatrix();
88.   translate(x1, y1, z1);
89.   translate(v1.x, v1.y, v1.z);
90.   rotateZ(the);
91.   rotateY(phi);
92.   noStroke();
93.   fill(strokeColour);
94.   box(weight, weight, p1.dist(p2)*1.2);
95.   popMatrix();
96. }