Ok, so you were as excited as I was to update my iPhone 5 to the latest IOS version 7. Upon clicking the update button you get a popup with this notification:
There are purchased items on the iPhone *** that have not been transferred to your iTunes library. You should transfer these items to your iTunes library before updating this iPhone. Are you sure you want to continue?
A few iTunes versions ago you would see your device in the left hand side, but seems Apple has gone the way of Microsoft and they now love to continue re-arranging the GUI (graphical user interface) so we have to search for something simple. The above screenshot is current as of September 2013...so here's the skinny:
Go to iTunes menu bar (top left) and select File>>>Devices>>>Transfer Purchases
What would be even more intuitive is for Apple to include button or link to do this automatically while the warning popup is glaring you in the face.
Hope this helps someone.