In my experience, adding buttons is a totally random process where you may or may not get your button the next time you start Rhino, depending on the phase of the moon and the relative humidity on Mars.
For example, recently we had a case where a button had lost its image. No matter how we tried to re-assign the image to the button, it did not show up the next time Rhino started. IMHO toolbars and RUI files are terminally broken.