Thank you for your quick response. Whoever is in charge of the docs might want to update this page as in the "Filtering the results" section it states...
Another common use case is returning just those songs that are available via a particular partner catalog. For instance, seeing all of the songs that are available on Rdio or 7digital. To do this, you must add a bucket= parameter, and name the catalog that you'd like the results from. Additionally, you can choose to restrict the results, only returning items available within that catalog. To do this, you must also add the limit=true parameter. Here are some examples:
(Note that the rdio-us-streaming song catalog used in this tutorial has been superceded by the rdio-US track catalog)
Return The Postal Service songs with Rdio IDs if they have any
http://developer.echonest.com/api/v4/song/search?api_key=FILDTEOIK2HBORODV&artist=the+postal+service&results=15&bucket=id:rdio-us-streaming
Cheers.