The trick here is that Dropbox has introduced this feature pretty late and only if you run the installer now (on a new machine perhaps) you'll notice the featur

The trick here is that Dropbox has introduced this feature pretty late and only if you run the installer now (on a new machine perhaps) you'll notice the feature is there because the installer will ask you if you want to sync everything or selectively. If I remember correctly, the older clients don't update to new versions automatically (or at least this was the case a year ago) and you will need to download a new client to make this feature available.