My USB serial port /dev/ttyACM0 would not appear in the Maple IDE under Ubuntu 9.10 Karmic Koala unless I ran the IDE as root. I did not want to do that. This is what I did to fix it.
The problem is that Ubuntu defaults the /dev/ttyACM0 device to be in the "dialout" group, but the IDE needs it to be in the "plugdev" group. You can manually "sudo chgrp plugdev /dev/ttyACM0" but when you unplug and replug, it will go back to the "dialout" group. This is because the plugging is handled by the Linux udev system.
udev's config files are in /lib/udev/rules.d/ . The particular config file causing the problem is /lib/udev/rules.d/50-udev-default.rules. This can be overridden by creating a file in /etc/udev/rules.d/ . I created a file called /etc/udev/rules.d/50-udev-tty.rules, and put the following lines in it:
# to make Maple hotplug USB tty work
Then I unplugged and plugged the USB cable to the Maple board, and restarted the Maple IDE. The board appeared in the Serial menu!