I watched your video and it is a good reuse of old hardware.  What you're trying to build is known to me as a CNC 2D plotter.  Other examples are the Makerbot Unicorn (http://www.makerbot.com/blog/2010/09/21/the-makerbot-unicorn-pen-plotte…) or something like the Eggbot (http://egg-bot.com/) or Spherebot (http://pleasantsoftware.com/developer/3d/spherebot/).  The latter two draw on cylindrical surfaces.  What all three have in common is that they use G-code to control the machine.  I have built my own version of the Eggbot/Spherebot from 3D printed parts, threaded rods, an arduino and some motors. (http://www.thingiverse.com/thing:7988) There are a few other 3D printable alternatives.  The way these work is that you make a vector graphic in InkScape (free software) and using a plugin, you convert the vectors to tool paths which are then converted to G-Code. The G-code is sent to the Arduino that controls the motors. The arduino has a sketch loaded that understands the G-code. A simple GcodeSender app runs on the computer to simply send the lines of G-Code to the arduino.

You may want to look into how those work and make changes to be compatable with your specific hardware.

If you really want to use MRL, you would either have to put the MRLComm.ino sketch on your arduino and then run a python script that replicates what your processing script does now.  The other way to use MRL is to leave your own Arduino sketch in place and have MRL talk to it using the Serial service. I'm assuming your Processing script uses serial to talk to the arduino now. You would still have to use the Python/Jython in MRL to send commands over that Serial service connection.

If you want to learn Python, here is a class I used to learn it myself since I already knew Perl. https://developers.google.com/edu/python/