Maybe your comport-numbers are too high.
A quote from the trouble shooting guide:
On Windows, the COM port assigned to the board may be too high. From zeveland:
"One little note if you aren't able to export and your USB board is trying to use a high COM port number: try changing the FTDI chip's COM port assignment to a lower one.
"I had a bunch of virtual COM ports set up for Bluetooth so the board was set to use COM17. The IDE wasn't able to find the board so I deleted the other virtual ports in Control Panel (on XP) and moved the FTDI's assignment down to COM2. Make sure to set Arduino to use the new port and good luck."
On the Mac, if you have an old version of the FTDI drivers, you may need to remove them and reinstall the latest version. See this forum thread for directions (thanks to gck).