I took a quick scan through the MANUAL which was avaible online at the DeLorme site, and did you do this step?
QUOTE
To Modify The Interface Settings
Use the following steps to change the serial port and USB settings on the device. The setting you choose will be based o the type of cable you are using to connect the device to your computer, your computer's operating system, and
if you are trying to access information on the removable media (SD) card.
1. Ensure the Device Setup Page is displaying.
2. Highlight the Interface option and press ENTER.
3. If you are connecting the device to your cmputer via serial cable, highlight ENABLE SERIAL PORT
and them press ENTER. A check mark in the Enable Serial Port check box indicates that the serial
port connection has been activated.
OR
If you are connecting the device to your computer via a USB cable, highlight the USB Setting drop down list and press ENTER. Then highlight DeLorme or Mass Storage and press ENTER.
If you didn't the USB port in the GPS may not be activated so the computer would see no USB device connected.