Once you load the UI web app in your browser for the first time, it will ask you to provide the IP address or hostname for your PFC2's Raspberry Pi. When you do that, the UI will try to connect to the CouchDB REST API on your Raspberry Pi. If that is sucessful, you can click the "Save" link up in the top right corner and begin interacting with the UI.
To experiment with the latest UI code, you will need to retrieve it from the OpenAgInitiative/openag_ui, build the project into static files using grunt, install the files as a CouchApp on your Raspberry Pi, and then visit the CouchApp's url. The CouchApp url might look like http://raspberrypi.local:5984/app/_design/app/_rewrite.
Spaghet, Gordon, Motty, and I have written notes about this process in some other forum posts and as Gists on GitHub: