In Linuxppc 1999 (also known as R5), a different X server is used. 
Therefore the option[num] for mouse buttons goes away. The qick work 
around is this:

Under bootX, add the following to the kernal arguments:

adb_buttons=113,115

This makes the help button your middle mouse click and the home 
button your right mouse click (those buttons are on the right side 
over by the number key pad).

You can use any keycode you want instead of 113 and 115, but I don't 
know any others off the top of my head except 99,120 which is F1 and F2.

The below solution was also found in the linuxppc-users mailing list archive, 
but I have not tested it's truthfulness:

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If I understand this correctly, this is not a question of R4 vs R5, but
one of Xpmac vs XF68_FBDev. It is related to R4 vs R5, because in R4 the
default X server was Xpmac, and in R5 it is XF68_FBDev.

In Xpmac, you have automatically the two emulations:
1. button 2 = opt+2 and button 3 = opt+3
and also
2. button 2 = opt_left+click and button 3 = opt_right+click

I prefer the second one, because it make things like ctrl-shift-button3
possible, but others find the first one more convenient.

Whereas both emulations work in Xpmac, in XF68_FBDev you need
collaboration of the kernel to achieve this. By default, you get 2 keys
(without modifier key) acting like button2 and button3. You can choose
these keys by giving "adb_buttons=xx,yy" as kernel argument, where xx
and yy are the (raw console) keycodes of the 2 buttons. If you want one
of the 2 emulations as in Xpmac, you have to compile a patched kernel.
The corresponding kernel patches have been around for quite a while, but
AFAIK, none of them has made it into the kernel sources. I am using a
patch coming from Franz Sirl originally that implements method 2 above.
I have two versions of this patch, for vger kernels 2.2.10 and 2.3.5
(works also for 2.3.6), on my site
http://www.maths.univ-rennes1.fr/~costabel/linux/boot/.

This patch gives you method 2 above, if you put "adb_buttons=2,58,124"
into your kernel args. The 58 and 124 may vary with your keyboard.

I am also using Xkb in XF68_FBDev, i.e. the XkbDisable line is commented
out in XF86Config. I don't remember if this is necessary for the mouse
button emulation to work, but it is possible that without this, the opt
modifier keys don't work, at least on my 6400.

The adb+buttons=keycode,keycode does seem to works fine. You do need to know
the keycodes though.

To find the keycodes, use
showkey --keycode

it is pretty self explanitory. However, there is a twist. You don't seem to be
able to run showkey in X, so you have to go over to one of the other terminials 
to do it. From the X login screen, hit the

Ctrl-Option-Command-F1

key combination. You get to a terminal with an old-style unix login prompt. You 
may need to hit return a couple of times to see the prompt. Login there and use
showkey. To get back to X, use

Ctrl-Command-F?

Notice that Option is missing from the combo. Notice also that I don't know 
exactly which F key to hit, since this depends on your system configuration.
There are only 12 of them, so it shouldn't take you too long to find the right
one ;-) I can guarentee you it won't be F1.

Good luck
--Mike gertz@anl.gov

Another solution is to stop using the option key for ALT. 
Create an .Xmodmap file in your home directory with the line:

clear mod2

Then reboot the xserver. Voila! option-2 or left-option-click are again
button-2, and option-3 and right-option-click are again button-3

There is also a system Xmodmap if you want to
change it system-wide. you can find the location of the system Xmodmap by
reading the file /etc/X11/xdm/Xsession. The current system modmap is at 

/usr/X11R6/lib/X11/xinit/.Xmodmap

Of course, it may not, by default, exist. It will, however, be read if it is 
there.