In addition to the online functionality available within the Mr. Voice application, your Online Key allows you access to some extra features of the Mr. Voice website. If you use the Login link at the top of the pages, and put in your Online Key, you'll be able to download songs directly from the website.
After searching for songs, you can use the Download icon on the right to download the audio file directly, or you can use the Shopping Cart icon to add the song to your cart. If you're using a cart, after filling it up with the songs you want, you can use the Download link at the top to download a .ZIP file. That file will contain all of the songs you've added. But, more importantly, it's a Bundle file that you can import into your local copy of Mr. Voice via the Songs->Add Songs From Bundle menu. Doing that will add the songs, with all of their title, artist, and other information in one shot - very convenient!
There may be more online functionality added for registered users in the future - keep checking back, and send suggestions my way!