Play songs from iPhone in iTunes?

Hi all,

I've been trying to get my head around this, but I just can't get it to work. During music lessons, my teacher is able to play back songs on his iPod through his macbook (using iTunes, he selects the connected iPod and then browses to the song and is able to play it). I have tried to achieve the same thing, but for some reason all songs on my iPhone are greyed out and unclickable (thus unplayable).

Now I must admit that I have only recently purchased my MacBook, so the problem may be caused that the iPhone was not originally activated on this iTunes. I have however made sure that I authorized the Macbook to use my iTunes account. Also note that all songs currently on the iPhone are synched from my MacBook, afaik. I have also tried to perform a reset and completely reconfigure the iPhone, but this also did not fix the problem.

Does anyone have a clue how I could fix this problem? It would be great if I could use this function, cause that would mean I won't have to bring my huge external with me on every trip.

Cheers