Minecraft wiki entry for Migrating_saved_game_data_to_the_cloud, associated text: GUI
Creating symbolic links in Windows is easy thanks to a free application called Link Shell Extension, which will allow you to create a symbolic link via right-click in Explorer.
Note: If you are running Windows XP, you may need to install the Server 2003 resource pack in order to enable symbolic links, which is available here.
Move your Minecraft saves folder (located in %appdata%\.minecraft) to a folder in your Dropbox.
Right-click on this new folder and choose Pick Link Source.
Go back to your Minecraft folder's original location, then Right-click somewhere blank and click Drop As… → Junction,
and you are all set to pull your game data off the cloud.