I successfully followed Melka's instructions that have been posted on the bricktable blog (update: the bricktable blog instructions have been updated, you should follow those instructions rather than my version here).
You'll need access to a windows machine to complete this procedure, since the MProg software is windows only.
I'm going to paste his instructions here too (I hope that's not treading on anyone's toes), and include some of my own notes inbetween.
you have to install Mprog and the D2XX drivers from FTDI (your arduino will still work with the arduino IDE with this drivers)
MProg 3.0 > http://www.ftdichip.com/Resources/Utilities.htm
D2XX Drivers > http://www.ftdichip.com/Drivers/D2XX.htm
1/ Install both, then run Mprog
I installed the D2XX drivers on my winXP laptop first, following instructions in this PDF. This part requires you to have the arduino plugged into the PC via the usb cable. NB. while following these instructions the 'found hardware wizard' will run twice, this is normal, just point it to the directory that contains the drivers you downloaded both times.
I then installed Mprog on the same XP laptop, which was straightforward.
2/ Launch Mprog, then click on Device / Scan. You should see something like this appearing in the box down.
Number Of Blank Devices = 0
Number Of Programmed Devices = 1
3/ Click on Tools > Read and Parse. This will fill the boxes.
4/ Check the “use fixed serial number” box and change the value below.
To have your board recognized as a monome by monome serial, you have to enter something like
m40h-xxx (we used m40h-001)
I used the m40h-001 code too.
Choose whatever you want, I’m not sure what is different between the various IDs, for the moment I’m using m40h-001.
There is 2 protocols described on the monome site, and the 64/128/256 seems to be more complete (15 message IDs, 9 for the 40h protocol), so maybe you’d better use m256-xxx
5/ Click on File > save as
6/ Once saved, click on the flash icon (Program all existing devices, Ctrl+P).
It's the icon that looks like a lightening flash.
7/ Unplug / plug back your board from the usb port.
In my case, this meant unplugging the arduino's USB cable from my XP laptop and plugging it in to my macbook (where i have MonomeSerial installed).
8/ Run monomeserial, you should see something appear on the devices list.
You'll see the serial code you chose in the devices list (m40h-001).