Mine still showed up after rooting, however I
am having one issue with it. When I receive a notification when the screen is off, if I wake the phone, the widget isn't there. I have to turn the screen off and then on again in order to see the music controls on the lockscreen. So far I can only say for sure this happens when I've received Google Voice notifications.
My advice for your issue would be disabling the app, restarting the phone, then re-enabling it. Just something to try, I guess.
Where is the Google Music Lockscreen widget? I just looked for it. I'm not rooted....
When you have music playing in Google Music, you have access to quick controls (previous, pause/play, next) on the lockscreen. I'm also pretty sure that some security options cause them to not appear, in the same manner that a pattern unlock pushes them up to the top of the screen.