Can't transfer iTunes music to iPhone

I've had my iPhone for about a year, and my Macbook not as long. When I plug my phone into my PC, which is the computer I used when I got my iPhone, it allows me to transfer music over. But, when I plug my phone into my Macbook I can't transfer music over, but it shows up as my iPhone in iTunes. When I try to drag music over it doesn't let me drop it. Is that because I first used the phone with my pc? How can I get it to work with my macbook instead?

I'm a little confused. You say when you drag your music over? You Sync the iPhone to iTunes to transfer music, apps, etc. The iTunes Help file explains the procedure.

You can't directly drag music from the iPhone to the Mac, but there's a freeware program called Senuti (itunes backwards) that copies music from the iPhone and saves it on the mac. Then just drag the transferred music files into iTunes and they are added to the music library. BTW: It also works with iPods.