This question was asked here Android App base 64 public key two years ago, but the interface to the Google Developer Console has changed since then. Some documentation refers to http://market.android.com/publish/editProfile and everything else that the documentation states is there can be found instead by going to the developer console and then to Settings and then Account details. Not so the key. I have also seen documentation that says the key can be found on the "Services & APIs page in the Developer Console" - Under settings, there is an API section but this is for interfacing to the developer console and you have to "create a project" to gain access. I don't think this is what I need either. There is no services section.
My problem was that I wanted to test before the app was uploaded on to the play store. By starting to create a Play listing for the app (no need to fill anything out) it is shown as Draft. At his point the Services & APIs section becomes available and the key can be copied.
UPDATE: http://developer.android.com/google/play/licensing/licensing-reference.html states that you can no longer test Licencing unless it has been published to at least Alpha.
So, Ignore any documentation that states you can test unpublished apps. Also, ignore the text in the developer console under where you can set a test response in the settings that says: "All accounts listed above will get the Licence Test Response. The account owner (but not the other test accounts) will also get this response for applications that have not been uploaded to Google Play yet."