If the window cannot be made active, the pointer will be a large "X". You may also see other symbols, such as the clock face that indicates something is in progress.
Motif assumes a mouse with three buttons, but if your mouse has fewer buttons, you will probably be able to achieve the same results with some combination of buttons and actions. For example, a two button mouse may simulate the third button by having both buttons depressed. At the moment, the Empress GUI Builder only uses the left and right (first and third) buttons.
The most important mouse-button operation is selection (the equivalent of hitting "Enter" on the keyboard). This is usually done by clicking the leftmost button.
The basic mouse actions are moving a pointer around the screen, and using the buttons. Depressing and releasing the select button is called "clicking". Several clicks in quick succession are called "double" or "triple" clicks, depending on the number. How quickly you will have to push the button to get multiple clicks, (as opposed to a series of single clicks), will depend on your installation.
The mouse can be used to move items around the screen. To "drag" an item, position the mouse pointer within the item to be moved, depress the select button and hold it down while moving the outline of the item. When the object reaches the desired location, then release the button.