Since you may have other devices on your system, Eclipse cannot work this out.
On your desktop PC you need to Run the 'MC50 Selector' program which is normally found in the Programs>Navigator Systems>MC50 group on your start menu.
The display shows the name you gave the scanner 'fred' in this case, the type of device - ie (MC50), and the storage location defined by the synchronization software.
Select the device that you are using and then press 'Save selected for Eclipse' and close the window.