You call the "Selections" script by
double-clicking its thumbnail in the Effects palette.
The Commands panel occupies the right side of the dialog box.
On the left side there is a list of available color and alpha channels (the both types may be used for selection purposes).
When working with large files the "Selections" dialog may open very slow. To speed up the script check the "Turbo"gadget at the dialog bottom.
Since in the "turbo" mode, the left panel is disabled, there appear several extra commands in the right panel for loading the color channels to selection (screenshot).
