OSX Lion, 64-bit dmg install error - no mountable file systems (no replies)

I downloaded the most current 64-bit dmg for Mac OSX (mysql-5.5.17-osx10.6-x86_64.dmg), but I get a "no mountable file systems" error message when I try to install the .dmg. I've searched but haven't been able to find an answer.

I am such a newbie that newbie doesn't even beging to describe me. What can I do? FWIW, I'm running OSX Lion on a new intel based MacBook Pro. Any help/advice would be much appreciated!