Circle is ready, but he looks flat, something like without volume, just a red spot. If you look our original button you can see that he have highlights and shadows. Exactly these artistic parameters give to him this volume.
To add volume to this red spot we need to color it with suitable color gradient which give to the circle the right shadows and highlights. You can make this with Interactive Fill Tool instrument from Toolbox pallete.

Next left click on the circle to select, click again and hold, drag diagonally upon the circle and relase. Now you have start and end points for your gradients. These two points can be described with position and color. The exact gradient will appear between them. Also you have scroller at the middle of gradient path with which you can specify the how linear have to be the gradient and on which stroke must be the most gradiented part.
Now you can move the two points upon the circle to position them and get better result. When you finish click on the two colors in Property Bar one by one to select the right colors. For top-left point this need to be the active (in our case this is red) color and for bottom-right black color. Next click on Radial Fountain Fill to make circle gradient. Make sure that your internal point of Radial Fountain Fill is the red point and the external is black point. If no, change color of the points.