Trouble running "Universal Windows app that consumes data from the Azure Marketplace" sample

I am trying to run the sample titled "Universal Windows app that consumes data from the Azure Marketplace". I am not sure how to enter the credentials for the Zillow APIs.  I obtained a key from Zillow, but when I enter it per the instructions in the sample, I still get an unauthorized (401) error.  

The instructions say  Insert that key into the SampleDataSource file of your project in place of the string "Your account key goes here".

So I replaced my key in this line of code:

request.Credentials = new NetworkCredential("<Your account key goes here>",
                                    "<Your account key goes here>");

Am I missing something else?