De acordo com a página da Wikipedia sobre DMGs :
In Windows, most dmg images can be opened using several other programs
such as Acute Systems TransMac, HFSExplorer, 7-Zip, UltraISO or
IsoBuster. MacDrive[9] can also mount simple dmg files as drives under
windows, but not sparse disk or encrypted dmgs.
Eu sugiro que você tente 7-Zip como você pode usá-lo gratuitamente e é um software bastante sólido.
Para qualquer usuário de Linux que esteja se perguntando sobre isso, a mesma página da Wikipédia indica:
In Linux and possibly other Unix flavors, most .dmg files can be
burned to CD/DVD using the program cdrecord or directly mounted to a
mountpoint (e.g. mount -o loop,ro -t hfsplus imagefile.dmg
/mnt/mountpoint).
PowerISO is a proprietary, cross platform, Freeware CLI tool that runs
on Linux; it can convert most proprietary image formats to ISO, and
newer versions support .dmg.