Please Help a Guy that isn't technically incline with gadgets!

Ok, so I'm surprised myself to find this forum but maybe someone here knows what I need to do. I bought a Griffin iTrip car charger&FM Transmitter from Target. The salesman said it would work for my iphone(iphone4s). So I want to know what's missing in this puzzle? Do I need another cable? I doubt that because all the holes and plugs are already filled. I have a stock head unit in my 2002 Lexus is300. :confused: I downloaded two applications for the iTrip already. A-Ha,& iTrip. And last part to this puzzle is I'm a premium member to "Spotify"music , that I have downloaded a lot of music on my iPhone4s already and it has a feature of listening to music Off-Line and I'm wanting to hear that in the car and iTunes would be cool too. Or maybe Google plus music? Whoever can help me is owed a lot of gratitude by me. :D

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