UICheckbox not accepting input.

[MaskedPixel]

Posting to maybe help someone...

Make sure your "Radio Button Root" is on the correct layer.

While pretty obvious in hindsight, it took a while to figure out as that behavior isn't the most intuitive.

[satrio]

I had the same problem with "input field".

It worked if I put the collider and script on the label object as in the example provided with NGUI.
My problem was that I had grouped the input field with some other objects to organize under an empty game object.
This empty gameobject needs to have the same layer as the rest of the panel.

Thanks to your post, I could find the problem! 

[ArenMook]

Use ALT+SHIFT+N to add child game objects. When you do that, they will assume the layer of the parent. There are other useful shortcuts: ALT+SHIFT+S to add a sprite, ALT+SHIFT+L to add a label, etc.