Instructions
Connect a USB flash drive to the iMac. Open the "Disk Utility" location in the Application folder.
Select the USB flash drive in Disk Utility's side bar. Select "Partition." Set the "Volume Scheme" to "1 Partition" and set the "Format" to "FAT32." Click "Apply" to format the USB flash drive for use on the PS3.
Copy the files to the USB flash drive from your iMac that you wish to bring to the PS3. Organize the copied files into categories -- put videos in a folder named "VIDEO," and music into a folder named "MUSIC."
Disconnect the USB flash drive from the iMac. Connect it to an open USB port on the PS3.
Power on the PS3. The files on the USB flash drive will be available in their respective categories within the PS3 XMB menu system.