Adding gradient fills?

Hi,
i am new to photoshop and i am trying to follow the following link tutorial: http://www.photoshopstar.com/web-graphics/glossy-navigation-buttons/
i’m at stage 2, but i don’t know how i can add a gradient fill inside my buttona as the tutorial does not say. When i click on the gradient editor and select 1 of the presets i can’t seem to click on my button shape as it says, ‘Could not use gradient tool because the content of they layer is not directly editable’ so how can i edit it so that it becomes like the color seen that of the tutorial?

or have i done it but not realized it?
Thanks