Amount
Use the Amount setting to determine the amount of dust spots applied to the image.

Size
Size determines the size of the individual dust spots. The higher this value, the bigger the spots get.

Thickness
Using the Thickness setting you can determine how "thick" the spots of dust will be. The Dust effect will randomly add dark spots to each other and the Thickness determines what percentage in the range of transparent to black will be added each time.
Setting a low value here will make the dust spots less obvious but more smooth and is generally the best option. Setting it higher will bring more contrast into the dust spots.

Transparency
This setting determines the actual visibility of the effect, setting it to any version higher than 0% will mix the dust spots with the image.

Color range
Use these two color boxes to set which colors the spots may have. OldMovie will pick any color inbetween these two entries at random, using only lineair scaling so the hue will always be within the range specified by the two colors.
Click on the color boxes to specify another color using a the hosts' standard color dialog, if you want to use only one color for the dust then just set both colorboxes to the same color.