Opening dmg files

[R Skinner]

I'm checking out downloading the video files for the FreeBSD kernel 
course offered by McKusik, and I downloaded an encrypted file which I 
was able to decrypt, but I still can't actually play it. I don't believe 
I would have downloaded 650Mb of nothing, so how do I play it?

The format is .dmg which is apparently a mac cd format, but I can't view 
the files in it, and mplayer and others don't recognise it either.

TIA