Most of what you have there is superfluous code. Note that line 59 just won't work. With control events you can change variables, which you can then use in the draw() loop. Note that scale will scale everything, also the position and the controllers. So you may need a stategic placement of scale inside push/pop matrix calls.

The good news is that ControlP5 already magically connects a variable to a controller of the same name and sets it's value at the start and when moving the slider. See the following example. Also look at all the examples that come with ControlP5, they show very clearly all the possibilities of each controller and much more.

Code Example

1. import controlP5.*;
2. ControlP5 controlP5;
3.  
4. float scale = 0.92;
5.  
6. void setup() {
7.   size(779, 500);
8.   controlP5 = new ControlP5(this);
9.   controlP5.addSlider("scale", 0, 50, 20, 20, 100, 30);
10.   noStroke();
11. }
12.  
13. void draw() {
14.   background(2, 240, 222);
15.   pushMatrix();
16.   translate(width/2, height/2);
17.   scale(scale);
18.   fill(0);
19.   ellipse(0, 0, 10, 10);
20.   popMatrix();
21. }