First, make sure iTunes Match is enabled under your subscriptions in your account so sign in using your Apple ID and check. Second, "iCloud Music Library" must be enabled. Go to Settings, then Music to enable it. Upon completing both of the above, your music library should begin to download. If you have updated to IOS 8.4, the music interface is slightly different in that you will now use the Artist, Album drop list to enable "show all music" which is now called "Offline Listening". It must be turned off to show your entire cloud Library