Updating label on push notification callback (IOS, Titanium Studio)

I've implemented the call back for push notification in index.js so that I can notify the user when he/she is using the app.

For example, I have to increase the message count in a label when the call back is called. I tried having the label as global and setting the value on call back as below:

callback:function(e)
{
     Ti.App.messageLabel.text = 10
}

But, it was not possible to update the value of the label. Any help on how to set the value of a label from index.js or how others do it would be appreciated. Thanks.