In Command And Conquer World Builder you simply select a WAYPOINT and the place it where you would like the starting are to be then select Player # starting location in the menu.
The Mismatch error happens for diffrent reasons. 1: The players have diffrent versions of the game or mods installed. 2: Someine is using cheats. Or 3: a connection error, where one or more players lose a connection to the internet.