Trouble transferring music from my mac to my iPhone?

I'm trying to move a song from my Mac to my iPhone 4S using iTunes. For some reason when I try to drag & drop it, it doesn't drop. It's an original song that's not on iTunes, so I can't just go and rebuy it on iTunes on my phone.

More guys are using Mac iPhone transfer platinum program instead of iTunes. It is very easy to use but can solve your big problems. When your iPhone connected to Mac, all your iPhone files will appeared on this program. Then you can hit "Add File" button to transfer files from Mac to your iPhone with ease. Hope it can help you.