However having tested the application on Android and iOS it looks like the service is only available for Mac and Windows users at this point. Microsoft will have to work on making the service available to iOS users if they really want Xbox Music to be considered a serious competitor to services like Spotify.
If you have a compatible device, simply point your browser to music.xbox.com and get started. This will be most useful to Mac owners who have recently switched to a Windows Phone or if you have a tablet low on memory. Additionally it offers the ability to read music stored on your device SD card, just sign into your Microsoft account and your entire library should quickly synchronized.