Because "My Documents" contains a space, the quotation marks are required.
The easiest way to reference this is using the environmental variables, as follows: "%HomeDrive%%HomePath%\My Documents\Dropbox".
The good thing about Dropbox is that it is cross platform, plus available on the web and web-enabled smartphones. Dropbox is a nifty little tool that automatically syncs content to their servers, and allows you to access it both online and through a client on your PC. If you don't know what those are, then you're probably safe. jar customizations (such as mods) or texture packs. This guide also will only sync your saved game files, not any. If you wish to have separate directories (e.g., for other machines: Minecraft/Linux, Minecraft/Windows, Minecraft/Toaster, etc.), then you should create them on Dropbox and change the instructions here as needed. This guide assumes that you only want one integrated Minecraft save directory. 3 Copy your Minecraft saved data to your Dropbox folder.