XML : Adding a button?

I am using this external library in order to add effects to my button:

https://github.com/markushi/android-circlebutton

And I want to add text to my button. When I do the following:

  <at.markushi.ui.CircleButton      android:layout_width="100dip"      android:layout_height="100dip"      app:cb_color="#99CC00"      app:cb_pressedRingWidth="30dip"      android:text = "hi"      android:layout_gravity="center"      />    

The "hi" doesn't show up. How can I add text to the button?

Thanks,

Ruchir