why does installation of digital editions refuse to end?

I have tried to install Digital Editions 4.5.2 on my Mac Pro (OSX 10/9/5).  The installer gets to "installation", then says "waiting for other installations to complete".  It has now been waiting almost 24 hours.

There seems to be no way to interrupt the installer.  I have closed all other programs on my computer, so no other installations seem to be happening. I have tried to restart, but the request is refused by the installer.