If you are using Windows Vista and a Windows Mobile 6 Standard device, it is possible that you have found that Windows Mobile Device Center (WMDC) does not support file synchronization. Oddly enough, this option is available in ActiveSync (WMDC’s predecessor). Fortunately, Microsoft has recognized that there is a problem and a fix is on the way.
According to Microsoft,
Currently, Windows Mobile 6 users can synchronize files between PCs and smartphones with Active Sync 4.5. Microsoft will update Windows Mobile Device Center (WMDC) on Windows Vista PCs to support file synchronization with Windows Mobile 6 by mid-June. We appreciate our customers continuing to use ActiveSync 4.5 for file synchronization until this update is available and are looking forward to offering the WMDC synchronize option very soon.
Contrary to what is suggested in the Microsoft statement, ActiveSync will not work on Vista. You can either hold off upgrading to Vista until a fix is available or live without this feature if you run Vista.