TL;DR ln -s /opt /root
Working with a Digital Ocean droplet in Cloud9 is awesome and easy - except that Cloud9 creates it's workspace in the wrong directory (/root instead of /opt - at least for a MEAN droplet).
Here's an easy trick to get your application files into the workspace (so you can open them in the Cloud9 editor instead of having to navigate to them through the terminal and open them with VIM - ain't nobody got time for that).
Run the following command in a Cloud9 terminal.
ln -s /opt /root
This will create a soft link from the root directory (where your workspace is) and the opt directory (where the MEAN app is).
Now you can open all your MEAN files using the awesome Cloud9 editor!