I know almost nothing about any Apple product so the following is just from searching the net.
According to http://en.wikipedia.org/wiki/Apple_Disk_Image
"Apple disk image files are essentially raw disk images (i.e. contain block data) with some added metadata, optionally with one or two layers applied that provide compression and encryption
So not surprising that you are not succeeding when 'dd'ing the files to SD cards.
".dd/.cdr (raw image)
These files are raw dumps, as the dd program uses them.
You can convert .dmg files to these on the Mac, using the following command:
hdiutil convert some.dmg -format UDTO -o some.dd
hdiutil uses the suffix ".cdr" for these files, but in fact the result is a raw dump that can be used with standard, cross-platform tools such as dd. This is why we prefer the ".dd" suffix.
To write a raw dump to a physical disk:
CAUTION: the following commands are dangerous. You might accidentally wipe your hard disk.
Do not use this if you do not know what you are doing.
On the Mac, it can be written with
dd if=some.dd of=/dev/rdiskX bs=2048"
So try converting and then 'dd' ing to an SD card.