Put your API-keys in the appropiate *.properties key. Use the sample files to see how the files have to be structured.
Freebase and Youtube API-keys have to be obtained from the google developer console. There you need to enable YouTube Data API v3 and Freebase API. To do so, navigate to "APIs & auth" --> "APIs" and switch the status of these APIs to on. Now you are allowed to use you API-Key. To actually get the key, click on "Credentials" and create a new key for browser applications. Copypaste it into the freebase.properties file like this
API_KEY = your_api_key
in the same folder the freebase.properties.sample can be found.
Be advised you must not push your api-key to a public repository!