There are many ways. The best would be to use the White-To-Transparent Gradient.
Alternatively, create a new Layer on top of it. Select the part where you want the shine. In the new layer with the selection still made, create a White to Black gradient. Change the Blending Mode to Screen. Now just adjust the opacity of this layer till you get what you like.
I tried it on your PSD. It's not too effective, but that's because your uhhh "ball" looks weird. Believe me, it works. 