Minecraft wiki entry for Saved_data_Dropbox_guide, associated text: On Windows XP, the Dropbox folder is located inside the user's "My Documents" folder. The easiest way to reference this is using the environmental variables, as follows: "%HomeDrive%%HomePath%\My Documents\Dropbox". Because "My Documents" contains a space, the quotation marks are required.
On Windows 7 and 8, the default Dropbox folder location is C:\Users\username\Dropbox. Use the environmental variables %HomeDrive%%HomePath%\Dropbox.
On Windows 10, the Dropbox folder is labeled as its own item in the sidebar of the File Explorer window.
On Linux, the default location is in the user's home folder. For example, in Ubuntu, that folder is located at ~/Dropbox
On MacOS, it should also be in the user's home folder.