Check the UI sample scenes, all of them bind buttons.
The gist of it - you have 2 options.
1. If you want to do it from code,
var myToggleButton : tk2dUIItem;
function OnEnable() {
myToggleButton.OnDown += ButtonDown;
}
function OnDisable() {
myToggleButton.OnDown -= ButtonDown;
}
function ButtonDown() {
Debug.Log("Button pressed");
}
2. If you want to do it from the UI, drag your controller behaviour into the "Send Message.Target" slot on the button. (eg. 3D UI sample, button in the hierarchy). You then select ButtonDown on the OnClick message. You get the same result as above, without having to write any code. Its just preference which way you want to work with this.