At this time, CarPlay iOS doesn't support the music by streaming (but support natively Spotify). The Music app can show you the different artists or Album of your library Apple Music or iTunes Match but you can't play the songs (that explain why you can have “No content” displayed, in reference to No content to play locally).

If you want to play your songs, you just need to download them on your iDevice (from the Music app of iOS) or transfer your music with iTunes to your iDevice

You can play some of your songs downloaded locally from the iOS Music app for a offline listening.

The Now Playing app of CarPlay iOS support the control of the playback for the songs played from the iOS Music app. You can launch one of your song from a playlist for example, and launch Now Playing app of CarPlay iOS to control the playback.

With Now Playing app you can :

• Play/Pause the current song
• Play last/next song
• Change the playback position

At this time, Apple restrict the access to the library in the cloud, but when a solution will be available, an update on Cydia will be available in a short time.