Você precisa executar o comando de extração novamente no arquivo 2.hfs,
$ 7z x file.dmg -oOUTFOLDER
$ cd OUTFOLDER
$ 7z x 2.hfs
7-Zip [64] 9.20 Copyright (c) 1999-2010 Igor Pavlov 2010-11-18
p7zip Version 9.20 (locale=en_US.utf8,Utf16=on,HugeFiles=on,4 CPUs)
Processing archive: 2.hfs
Extracting WinZip
Extracting WinZip/.background
Extracting WinZip/.DS_Store
Extracting WinZip/.HFS+ Private Directory Data
Extracting WinZip/.Trashes
Extracting WinZip/Applications
Extracting WinZip/WinZip.app
Extracting WinZip/[]
Extracting WinZip/.background/backgroundImage.tiff
...