The least painful solution is to get a three-button mouse, but until
then, you need to enable mouse button emulation, and then choose
keyboard keys to use instead of mousebuttons.
I use F1 for the middle key and F2 for the right. Those are probably
not the best choices, as there are applications that need F1 and F2
for their own purposes. But to illustrate:
$ su
# cd /proc/sys/dev/mac_hid
# echo 59 > mouse_button1_emulation
# echo 60 > mouse_button2_emulation
# echo 1 > mouse_button_emulation
Keycode 59 is F1, 60 is F2. The "echo 1" enables the emulation.
(This was all from memory, I'm not sure it's exactly right.)