MacDropAny is pretty straightforward. First, you run the app and choose the folder (outside of your Dropbox) that you want to sync. Once selected, you’ll be asked where you want to place a symbolic link to that folder. You can choose any location in your Dropbox. For the last step, just name your symbolic link and it’ll be placed in your Dropbox. To add another folder, just run MacDropAny again.
For the same functionality on Windows, check out Dropbox Folder Sync.