You cannot open a CS6 INDD file in CS5. It needs to be saved back via IDML.
I also received an IDML saved back from CS6, and when I tried to open it, I
received a "cannot handle the request because a modal dialog or alert is
On Tue, Aug 19, 2014 at 10:25 AM, Bob Levine <email@example.com>
Close the dilaogs...
If necessary restart ID or reboot the computer.