You should be able to just search for Smart Glass in your device's app store, but here is a link to the xbox.com page that has links to all the other locations:http://www.xbox.com/en-US/smartglass#get-smartglass
There is also a support link at the bottom in case you have some issues.
Just download the app, open it up, connect it to your Live account, for me (on Android) it just opened up a page that had the Karaoke game linked there. I clicked on the + icon at the top of the screen and then added one of the Free songs of the day. The achievement came up a split second later.