Put your API-keys in the appropiate *.properties key. Use the sample files to see how the files have to be structured.
Be advised you must not push your api-key to a public repository!
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.
Last.fm keys can be requested on their website. Simply follow their instructions. Add both the api-key and the secret key to a file called lastfm.properties formatted like this
API_KEY = your_api_key
To get a Flickr API-key you need a YAHOO! account. (Create one, ) Log in and visit the YAHOO! API page and click "Request an API Key". Add both the api-key and the secret key to a file called flickr.properties formatted like this
API_KEY = your_api_key
SECRET_KEY = your_secret_key