MRL currently uses create-labs libraries to interface with serial info - They did a great job of normalizing the Java and doing a full build of 6 types (windows 64/32, Linux 64/32 & Mac 64/32)
Unfortunately the current Jar is filtering out ttyACMx which is the identifier for serial devices for Arduino Uno & Mega..
A easy work-around is to make a symbolic link like this :
ln -s /dev/ttyACM0 /dev/ttyUSB0
you might need to chown or chmod the new link to give it the appropriate permissions
After I did that I could connect to a Mega right away !