why not using a toggle instead ?
there is a working example on the controlP5 Website

import controlP5.*;
ControlP5 controlP5;

int myColorBackground = color(0,0,0);
boolean toggleValue = true;

1. void setup() {
2.   size(400,400);
3.   smooth();
4.   controlP5 = new ControlP5(this);
5.   controlP5.addToggle("toggle",false,100,160,20,20);
6.   controlP5.addToggle("toggleValue",true,100,240,20,20);
7. }
8.  
9. void draw() {
10.   background(myColorBackground);
11.   if(toggleValue==true) {
12.     fill(0,255,0);
13.   } else {
14.     fill(255,0,0);
15.   }
16.   rect(0,0,width,100);
17. }
18. void toggle(boolean theFlag) {
19.   if(theFlag==true) {
20.     myColorBackground = color(100);
21.   } else {
22.     myColorBackground = color(0);
23.   }
24.   println("a toggle event.");
25. }

if you still wanna use a button. you can do it like this :

1. import controlP5.*;
   ControlP5 controlP5;
   boolean val = false;
   
   void setup(){
     size(100,199);
     background(0);
     controlP5 = new ControlP5(this);
     controlP5.addButton("achsen",0,10,10,80,19);
   }
   
   void draw(){
   }
   
   void achsen() {
     if (val){
       println("click1");
       val=!val; 
     }
     else {
       println("click2");
       val=!val; 
     }
   }
   

I don't want to use toggles because I want the text in the button and not below..

I'll try it out..

thanks for your help..

alright, so do it like the second example then. not sure if you cant change the way the text is shown though.