Music on sony ericsson?

[WillH]

I have a mac so can't use the disktophone cd. So I plugged my k55oi into my computer using the usb cable and i put a coulple of songs on. After, I took them off so i could put others on, and now when I drag and drop the MP3 onto the music section of the phone it transfers it then deletes it again. A message also came up saying 'phone not modernised.

HELP PLEASE!