I have a PowerBookG4, and even though I can see my BlackBerry's storage card when tethered with the USB cable, dragging files into the folders resulted in an error message. Drats!
Fortunately, I found that using bluetooth worked fantastic!
Here's the step-by-step of what worked for me.
Select the Music application on your BB.
Choose "Receive file via Bluetooth" via the menu (of course, you will need to first introduce your computer to your BB via Bluetooth if you haven't done so yet).
On your Mac, click the Bluetooth icon in the menu bar (by the Airport icon if you're not sure where to look), and select "Send File."
Use the awesome spotlight feature to search: mp3 and all the MP3 files on your computer will appear in one handy list. If you know the song title, or the artist, add that to your search to find exactly the file you want even quicker.
Select the file you want.
Then, click Send again in the Bluetooth panel.
Now, back to your BB to accept the file.
On my BB, for some reason, all mp3 files come in defaulting to the ringtone folder.
To get them to save to the Music folder, before accepting the file, I click the file folder, select Menu, select UP, select EXPORE, click the storage card, click the BlackBerry folder, select the Music folder, click MENU and then scroll up to "Select Folder."
Sounds harder than it is, although if your BB automatically knows where to put things you can skip that.
Then, accept the file and it transfers right over without a hitch!!!
Loving my first BlackBerry (with iBerry theme!).
Gayanna Work Less. Play More. "I am kind of a paranoiac in reverse.
I suspect people of plotting to make me happy."