here...

a being speed for x
b being speed for y

px is the paddle.

remember the right side of the paddle can only be hit when a is negative
the left side of the paddle can only be hit when a is positive

and for the upper side: b positive

the lower side I haven't done yet.

1.     // paint the paddle
2.     rectMode(CENTER);
3.     rect(px, 500, 50, 10);
4.     // inside paddle
5.     if (x < (px + 25) && x > (px - 25) &&
6.       y > 493 && y < 505) {
7.       b = abs(b) * (-1); // speed for y negative
8.       score = score + 1;
9.     }
10.     // right side paddle
11.     else if (x < (px + 27) && x > (px + 23) &&
12.       y > 495 && y < 505) {
13.       // must come from the right side
14.       if (a<0) {
15.         println("outside border paddle right");  
16.         // b = abs(b) * (-1); // speed for y negative
17.         a = abs(a);        // speed for x pos
18.         score = score + 1;
19.       }
20.     }
21.     // left side paddle
22.     else if (x < (px - 23) && x > (px - 27) &&
23.       y > 495 && y < 505) {
24.       // must come from the left side
25.       if (a>0) {
26.         println("outside border paddle left");  
27.         // b = abs(b) * (-1); // speed for y negative
28.         a = abs(a) * (-1);        // speed for x neg
29.         score = score + 1;
30.       }
31.     }