I'm not sure if this has been posted yet, but I've been searching for the best way to play backups via usb using a mac and came up with nothing. So I figured this out, and it was a shot in the dark, but it worked perfectly.

First of all I think you need to have your IOS249 updated (mine is 19, and I haven't tested for previous versions)

1. Go to disk utility and partition your hard drive into 2 partitions (both FAT-32).
-The top partition should be where you store backups and hold about 3/4 of your storage space
-The second partition should be called FAT and should store all of your HBC data (apps, wads, etc.)

2. Download WBFS manager for mac (a quick google search should find it)
-Use WBFS manage to format your drive (it will only format the first partition)
-Add your ISOs.

3. Download WiiFlow (again I'm not going to include a download link)
-Add it using your mac to the FAT partition in /apps/

4. Start your Wii
-HBC will automatically read from the FAT32 partition and WiiFlow will automatically read from your WBFS partition.

5. Enjoy!