Trackball devices with a lower and upper button on the side use the:
Lower button for the primary button
Upper button for the secondary button
Tips
To click, point to an object on the screen and quickly press and release the primary button.
To double-click, point to an object on the screen and quickly press and release the primary button twice.
To drag an object, move your pointer over an object on the screen, click and hold the primary button, move the object to a new location, and then release the primary button.
To drag an object using ClickLock, point to an object on the screen, click and momentarily hold the primary button, and then move the object. To drop the object, click the primary button.
To display shortcut menus, point to an object on the screen, and then click the secondary button.
If you prefer, you can assign Click or Right-click to another button. See Assign a new feature to a button to assign a feature to a different button.