Great information in both posts. I had to do a lot of research myself to get this working and I wanted to post some things that I found that was either buried in other pages or discovered through trial and error.
First off to find your device name enter "xsetwacom list devices"
This will list off the inputs for the tablet in whole. As an example I have a Wacom Intuos Pen and Touch Small, so my device list is:
Wacom Intuos PT S Pen stylus id: 9 type: STYLUS
Wacom Intuos PT S Finger touch id: 10 type: TOUCH
Wacom Intuos PT S Pen eraser id: 13 type: ERASER
Wacom Intuos PT S Finger pad id: 14 type: PAD
The id numbers didn't matter to me for mapping the express keys, but it does help to know which "device" is which. When I started this I was treating the whole bundle as one device.
Next to set the express keys enter 'xsetwacom --set "Your Device Name" Button +X "key x"'
This is where I had to experiment a little. My keys were different than Clover's. For instance my +1 is bottom left, +3 top left, +8 bottom right, and +9 top right.
Since I'm using the tablet to draw I set my keys to functions I want to do quickly. So I have them set to the keyboard shortcuts for zooming in and out, undo, and new layer.
Another trick I had to work out was how to enter multiple key combinations (e.g. Undo is ctrl+z). To achieve this just put another modifier at the end. The function then becomes:
xsetwacom --set "Your Device Name" Button +X "key x" "key y" "key z" "key etc."
These are the values for my device so you can see what it should look like without unnamed variables:
xsetwacom --set "Wacom Intuos PT S Finger pad" Button +1 "key ctrl" "key z"
xsetwacom --set "Wacom Intuos PT S Finger pad" Button +3 "key ctrl" "key shift" "key n"
xsetwacom --set "Wacom Intuos PT S Finger pad" Button +8 "Key -"
xsetwacom --set "Wacom Intuos PT S Finger pad" Button +9 "key shift" "key +"
While testing the buttons and values I set them to keys a-d just so I could click them to see if they were working and identify keys.
Finally once you get your keys mapped and you like the configuration save the command lines into a file an name it .xsetwacom.sh and put it into a bin folder in your home folder so you can keep it seperate from your normal directory. Add an executable command at the beginning
chmod +x $HOME/bin/.xsetwacom.sh
Then add the file to your startup programs and you now have expresskeys that will work even if you log out of your session.
Commands gathered from xsetwacom help and http://sourceforge.net/apps/mediawiki/linuxwacom/index.php?title=Tablet_Configuration#Pad
This is a little off topic of binding the buttons to click, but seeing as how this post is the first that comes up when doing a search for express keys I thought this answer would help those just beginning their search.
Best Answer
After some searching around, I found this http://ubuntuforums.org/showthread.php?t=1515562
It seems that the buttons are like a mouse's. So there are three buttons for left, right, and middle click, and four buttons are for scroll. Buttons 1 and 2 are the buttons on the upper left of my Intuos and the Buttons for the upper right buttons is Button 8 and 9.
You can get a better explanation from the link.