Two things:

• The name of your object instance is mijnknopje (not mijnknopjekruis)
• To call a function you have to include these: ()
  

So the correct line is:

1. mijnknopje.runknopje();

One general advice: press CTRL-T (autoformat) once in a while, it will make your text more readable.

Thank you it works.

Your code will run. In that sense there is nothing wrong with it. However there are some things that could improve in my opinion. These are all somewhat minor points, but if you are looking for educational suggestions, read on.

First, your shape is not centered around the main xy. To see this, add the following code to your drawkruis function:

1.     fill(255, 0, 0);
2.     ellipse(xkruis, ykruis, 10, 10);

Second, you have very long variable names which requires more typing, gives more opportunity for typos etc.

Third, classes are usually written with a capital letter to distinguish them, so the class would normally be called Kruis.

Here is how I would write your code:

1. Kruis k;
2.  
3. void setup() {
4.   size(500, 500);
5.   k = new Kruis(width/2, height/2);
6. }
7.  
8. void draw() {
9.   background(255);
10.   k.display();
11. }
12.  
13. class Kruis {
14.   float x, y;
15.   float w = 20;
16.   float h = 100;
17.  
18.   Kruis (float x, float y) {
19.     this.x = x;
20.     this.y = y;
21.   }
22.  
23.   void display() {
24.     fill(0);
25.     rectMode(CENTER);
26.     rect(x, y, w, h);
27.     rect(x, y, h, w);
28.     fill(255, 0, 0);
29.     ellipse(x, y, 10, 10);
30.   }
31. }

You will probably encounter a lot of questions like these as you learn Processing. The good news is they have already been answered. Usually finding the answer is quicker than asking each question. You can find answers by going through the examples that come with Processing, checking the reference or searching the forums. GUI elements are a topic that is covered by the examples that come with Processing.

See: Examples > Topics > GUI > Button (and others)

For more than 2 different screens, I recommend using a switch() in the draw() loop.

Also, seeing your whole program, I wonder if using classes is the best route. Classes are very useful for many occasions. For example if you want to have 1000 instances of Kruis all over the screen. However for a program like yours, classes may overcomplicate matter. Given your final idea, consider if you want to use classes or not. Because in Processing they are not required. For example your Kruis class could also be done using a displayKruis() function, see below.

1. void setup() {
2.   size(500, 500);
3. }
4.  
5. void draw() {
6.   background(255);
7.   displayKruis(width/2, height/2, 20, 100);
8.   displayKruis(width/4, height/3, 30, 200);
9.   displayKruis(3*width/4, height/5, 10, 50);
10. }
11.  
12. void displayKruis(float x, float y, float w, float h) {
13.   fill(0);
14.   rectMode(CENTER);
15.   rect(x, y, w, h);
16.   rect(x, y, h, w);
17. }

Another general tip. Always load things in setup (or the constructor of a class) and display them in draw. For example images, but also fonts. You currently load the font each time it is displayed (60 times per second), which will slow down your program. You only need to load the font once. After that you can use it.