If nothing is connected to my dicimila, then everything works fine, but if I plug in just an led it disconnects from USB and doesn't show up in the ports menu. It is connected to 9vdc current and the USB cable and computer (running xubuntu Linux) are both fine. Does anybody have any ideas?
Disconnect the 9VDC and try again 
Where are you plugging in your 9VDC? Is it regulated or unregulated?
What type of LED are you using?
I tried it, it still doesn't work and I am using an 8mm red led.
It seems to disconnect slower if more stuff is plugged in.
I am using an 8mm red led.
I assume you have a resistor in series with the LED, how big it that?
One theory is that you are drawing too much current with the LED that is temporally shorting out the supply (well putting a big load on it). That would cause the USB port in the computer to shut down and it won't start again until the computer is power cycled.
Plug a 6-12VDC 1-2A unregulated power supply to your arduino's 2.1mm power jack and set the PWR jumper to EXT PWR
If your LED draws more than about 20-40mA you cannot power it directly from the Arduino
Also as Grumpy_Mike mentioned you need a resistor to drive a 3.3V LED because the Arduino provides +5VDC
it's plugged into digital io 13, which is 3.3v. That led type always worked with my arduino before it broke.
The resistor is to limit current not voltage. You should have one to protect the Arduino if for no other reason. Try a different pin (with a 330 ohm resistor)
I plugged the led in with a 470ohm resistor, and it worked.
Thank you.