Normally, the middle button is used to insert annotations, so the problem is to simulate a middle button click if you don't have a middle button. Most, if not all, X servers that support two-button mice provide a way to do this. The most common method is to press both buttons simultaneously (an operation sometimes called ``chording,'' by analogy with playing a chord on the piano). Typically, you have an adjustable window of 50 to 100 milliseconds in which to press both buttons, so that you don't need exactly simultaneous clicks (which would be impossible to achieve reliably). Some X servers delay reporting a button-down event until a button-up event occurs; this approach makes chording less error-prone, but it means that the feedback that WAVE produces in response to button-down events gets delayed until it may no longer be useful. Another approach, more commonly used with one-button mice, is to simulate the middle button (and, if necessary, the right button) using a mouse button and keyboard key combination, such as clicking while pressing the or keys. Check your X server's manuals for information about which of these methods is supported. With a little practice, annotation editing can be tolerable with a button-impaired mouse.
Independent of the X server, WAVE also makes it possible to simulate a middle button click, by using the key (or the key on the numeric keypad). If you use this technique, you might also wish to use the and keys to drag the pointer and marker bars left or right.
Inexpensive three-button mice, trackballs, and touchpads manufactured by Logitech and many others are widely available for PCs, and are highly recommended if you do much annotation editing. Most are fully compatible with Microsoft two-button mice. Some users prefer trackballs for precision editing since there is no tendency for the pointer to move when clicking the buttons, as with a mouse.