Connect it with USB and then ''connect to computer" in the right tap-menu. Then you have 3 options. Choose Mass-storage.
The Imac will recognize it as a kind of usb-stick. you can drag and drop the music onto it. Have you emptied the folder, and it seems that you cannot copy anything to it, open disk utility in Mac and rewrite the partition.
There is also a kind of software called itunemywalkman. you can also use it to copy files from itunes to the serenata. There are no other options since itunes is a closed system for other devices than an ipod/iphone. B&O didn't develop their own software for mac.
Greets,
Michel