Choose the one that is closest to the fill you wish to create. If nothing is close, choose the simple black and white gradient, as it is easiest to edit. To create a new fill, click on the New button. The currently selected Gradient fill will be duplicated and a new icon created for your new custom gradient. Choose Linear or Radial from the drop down menu above the New button.
All color is controlled by pointers above the Color Definition bar.
The selected pointer is depressed, as shown at the left. The right pointer key is down, while the left pointer and the color well are up. Any change you make to color will apply to the selected pointer. The Pointer Well, the pointer to the far left, can be used to store a color which you will need often. Like the other pointers, it must be depressed to receive color.