What did you type in for the command prompt?
Did you try to use SyncToy.exe or the dedicated command line .exe - SyncToyCmd.exe ?
Here is an excellent primer on setting up SyncToy in a scheduled task. It runs on my computer every day.
http://www.howtogeek.com/howto/25046/schedule-synctoy-to-run-automatically-with-task-scheduler-in-windows-7/