You can get a list of available cameras with "Capture.list()".
And you can pass the name of the desired camera to the constructor of Capture.
for example:

1. myCapture = new Capture(this, width, height, Capture.list()[0], 30);

see: Capture

This is my script modified from an AR demo sample :

1. /**
2.   NyARToolkit for proce55ing/1.0.0
3.   (c)2008-2011 nyatla
4.   airmail(at)ebony.plala.or.jp
5.  
6.   最も短いARToolKitのコードです。
7.   Hiroマーカを用意してください。
8.  
9.   This sample program is most small sample as simpleLite.
10.   The marker is "patt.hiro".
11. */
12. import processing.video.*;
13. import jp.nyatla.nyar4psg.*;
14. import colladaLoader.*;
15.     ColladaLoader model;
16.     float angle = 0.01;
17.     char axis ='y';
18.    
19. Capture cam;
20. MultiMarker nya;
21. void setup() {
22.   size(640,480,P3D);
23.      //copy duff.kmz manually to processing's "data" folder or choose sketch->add file
24.    model = ColladaLoader.getInstance("iBook.kmz", this);
25.    //model = ColladaLoader.getInstance(sketchPath("../data/duff.kmz"), this);
26.    frameRate(20);
27.    model.scale(3.0f);
28.   //colorMode(RGB, 100);
29.   //println(MultiMarker.VERSION);
30.   cam=new Capture(this,640,480, Capture.list()[0], 30);
31.   //myCapture = new Capture(this, width, height, Capture.list()[0], 30);
32.   nya=new MultiMarker(this,width,height,"camera_para.dat",NyAR4PsgConfig.CONFIG_PSG);
33.   nya.addARMarker("patt.hiro",80);
34. }
35. void draw()
36. {
37.   if (cam.available() !=true) {
38.       return;
39.   }
40.   cam.read();
41.   nya.detect(cam);
42.   background(0);
43.   lights();
44.   nya.drawBackground(cam);//frustumを考慮した背景描画
45.   if((!nya.isExistMarker(0))){
46.     return;
47.   }
48.   nya.beginTransform(0);
49.   //fill(0,0,255);
50.   translate(0,0,0);
51.   rotateX(30);
52.   rotateY(90);
53.   //box(40);
54.   model.rotate(angle, axis); //rotates Model as an animation 
55.   model.draw();
56.   nya.endTransform();
57. }
58. void mouseDragged()
59. {
60.   //scales model bigger or smaller
61.    model.scale(1+(pmouseY - mouseY) * 0.01f);
62. }
63. void keyPressed() {
64.  
65.    //shifts translates model in all 3D axis
66.    if (keyCode == LEFT) {angle = 0; model.shift(-5.0f, 'x');}
67.    if (keyCode == RIGHT) {angle=0; model.shift(5.0f, 'x');}
68.    if (keyCode == UP) {angle = 0; model.shift(-5.0f, 'y');}
69.    if (keyCode == DOWN) {angle = 0; model.shift(5.0f, 'y');}
70.    if (key == 'f') {angle = 0; model.shift(-5.0f, 'z');}
71.    if (key == 'b') {angle = 0; model.shift(5.0f, 'z');}       
72.    if (key == 'x'||key == 'y'||key == 'z') {axis = key; angle = 0.08f;}
73. }

Still, one of my webcam gives me this problem :
Problem connecting Video output to Quicktime. .

And the other cam is also a problem,
Problem starting streaming...
could not open file mapping object ...
could not unmap view of file .

and only the default one (build-in atop screen ) can be used.

Seems like a conflict using the cams in Processing. In other app or my own Flash script has no such issue .

Cheers
-poserpro