Download the Xbox Music app from the Windows Store. It's a separate app from the Xbox Music app that comes bundled with the OS. It should allow for your music to sync from your Windows Phone to your S2.
Xbox Music | Windows Phone Apps+Games Store (United States)
For those asking why there's a separate Xbox Music app, blame the carriers. Carriers won't let MS update any part of the OS without their approval, that includes updates to even something as basic as Xbox Music. Microsoft created an independent Xbox Music app in order to bypass carrier approval.
I know, and it doesn't. Not to mention the app itself is an ugly slow mess, hence the last part of my previous comment. Reading through other threads here though it seems I'm not the only one with this type of issue.
Not sure what's going on then. It works well for me transferring playlists between my S2 and 920. The only catch is that the streaming setting needs to be enabled. Then when I sync songs I can just hit the download button on my 920 to store them internally.
I agree that the app is noticeable slower. However, it's still considered a beta so I'm pretty sure it will get better in time.