The problem is that although you have a method called mouseClicked() this is NOT being executed when the mouse is clicked but it is being called from the draw method (every frame)

In the code below I have changed the name of the mouseClicked method to isOver that returns true if the mouse is over the circle otherwise returns false. The value in the test has been changed

I have added a new mouseClicked method OUTSIDE the ClickBall class so that it will be executed on a mouse click. This method calls the isOver method for each ClickBall and adds 50 to the score if successful.

Another issue is that the balls move too fast

1. PFont f;
2. int xdirection = 1;  // Left or Right
3. int ydirection = 1;  // Top to Bottom
4. int value = 0;
5. Clickball ball1;
6. Clickball ball2; // Two objects!
7. Clickball ball3;
8. Clickball ball4;
9. 
   
10. void setup() {
11.   size(200, 200);
12.   cursor(CROSS);
13.   noStroke();
14.   f = loadFont("AgencyFB-Reg-48.vlw");
15.   ball1 = new Clickball(color(255, 0, 0), 10, 100, 2, 8);
16.   ball2 = new Clickball(color(0, 0, 255), 10, 130, 1, 7);
17.   ball3 = new Clickball(color(0, 255, 0), 10, 100, 3, 8);
18.   ball4 = new Clickball(color(0, 255, 255), 10, 130, 4, 9);
19.   // frameRate(10);
20. }
21. void mousePressed(){
22.   if(ball1.isOver() == true){
23.     value += 50;
24.   }
25.   if(ball2.isOver() == true){
26.     value += 50;
27.   }
28.   if(ball3.isOver() == true){
29.     value += 50;
30.   }
31.   if(ball4.isOver() == true){
32.     value += 50;
33.   }
34. }
35. 
    
36. void draw() {
37.   background(255);
38.   textFont(f, 36);
39.   stroke(175);
40.   line(width/2, 0, width/2, height);
41.   fill(0);
42.   textAlign(CENTER);
43.   text("Score: " + value, width/2, 60);
44.   ball1.bounce();
45.   ball1.display();
46.   ball2.bounce();
47.   ball2.display();
48.   ball3.bounce();
49.   ball3.display();
50.   ball4.bounce();
51.   ball4.display();
52. }
53. 
    
54. class Clickball {
55.   color c;
56.   float xpos;
57.   float ypos;
58.   float xspeed;
59.   float yspeed;
60. 
    
61.   Clickball(color tempC, float tempXpos, float tempYpos, float tempXspeed, float tempYspeed) {
62.     c = tempC;
63.     xspeed = tempXspeed;
64.     yspeed = tempYspeed;
65.     if (xpos > width-200 || xpos < 0) {
66.       xdirection *= -1;
67.     }
68.     if (ypos > height-200 || ypos < 0) {
69.       ydirection *= -1;
70.     }
71.   }
72. 
    
73.   void display() {
74.     stroke(0);
75.     fill(c);
76.     ellipseMode(CENTER);
77.     ellipse(xpos, ypos, 20, 20);
78.   }
79. 
    
80.   boolean isOver() {
81.     // Is mouse position over the circle i.e. dist < radius
82.     if (dist(xpos, ypos, mouseX, mouseY) < 10)
83.       return true;
84.     else
85.       return false;
86.   }
87. 
    
88.   void bounce() {
89.     xpos = xpos + xspeed;
90.     xpos = xpos + ( xspeed * xdirection );
91.     ypos = ypos + ( yspeed * ydirection );
92.     if (xpos > width - 20) {
93.       xspeed = xspeed * -1;
94.     }
95.     if (xpos < 0) {
96.       xspeed = xspeed * -1;
97.     }
98.     if (ypos > height - 20) {
99.       yspeed = yspeed * -1;
100.     }
101.     if (ypos < 0) {
102.       yspeed = yspeed * -1;
103.     }
104.   }
105. }