lines like 13 to 15 don't work

• you have to end a line with a ;
  
• you can't have a dot at the beginning of a line, you need an object before the dot, so the method after the dot knows to which object it's referring to

try it like this....

1. import controlP5.*;
2. ControlP5 cp5;
3. Textarea myTextarea;
4. void setup() {
5.   size(700, 400);
6.   cp5 = new ControlP5(this);
7.   Tab a1=cp5.addTab("extra");
8.   a1.setColorBackground(color(0, 160, 100));
9.   a1.setColorLabel(color(255));
10.   a1.setColorActive(color(255, 128, 0))      ;
11.   //
12.   Tab a2=cp5.addTab("default");
13.   a2.activateEvent(true);
14.   a2.setLabel("first tab");
15.   a2.setId(1)        ;
16.   //
17.   // a1=cp5.getTab("extra");
18.   a1.setLabel("Second tab");
19.   a1.activateEvent(true);
20.   a1.setId(2)    ;
21.   //
22.   myTextarea = cp5.addTextarea("text", "test", 100, 120, 110, 122);
23.   myTextarea.setPosition(100, 100);
24.   // myTextarea.setSize(200, 200);
25.   myTextarea.setText("some text to show in second tab");
26.   // use tabs name here not its label
27.   myTextarea.moveTo("extra");// OK
    
28.   // myTextarea.moveTo(a1);// ERROR 
29.   // cp5.getController("text").moveTo("second tab");// ERROR
30.   // cp5.getTextarea("text").moveTo("second tab");// ERROR
31. }
32. void draw() {
33.   background(0);
34. }

did a little house cleaning...

1. import controlP5.*;
2. ControlP5 cp5;
3. Textarea myTextarea;
4. void setup() {
5.   size(700, 400);
6.   cp5 = new ControlP5(this);
7.   //
8.   Tab tab1 = cp5.addTab("extra");
9.   tab1.setColorBackground(color(0, 160, 100));
10.   tab1.setColorLabel(color(255));
11.   tab1.setColorActive(color(255, 128, 0));
12.   tab1.setLabel("Second tab");
13.   tab1.activateEvent(true);
14.   tab1.setId(1);
15.   //
16.   Tab tab2 = cp5.addTab("default");
17.   tab2.activateEvent(true);
18.   tab2.setLabel("First tab");
19.   tab2.setId(2);
20.   //
21.   myTextarea = cp5.addTextarea("text", "test", 200, 120, 110, 122);
22.   myTextarea.setText("some text to show in second tab");
23.   myTextarea.moveTo("extra"); // use tabs name here not its label
24.   //
25. }
26. //
27. void draw() {
28.   background(0);
29. }