EDIT 23.07.2014: PROBLEM SOLVED!
The problem seems to have been related to 64-bit vs. 32-bit performance. I was having the same problem on my Macbook Pro as on my Mac Pro, so I tried checking the "Open in 32-bit mode" box (in the Get Info pane in the Finder), and on my laptop that led to SurroundPanner V5 working just fine in Nuendo 6. When I got back to the studio and tried the same thing on my Mac Pro, I got repeated crashes on startup in 32-bit mode, but through some combination of going back and forth between 32- and 64-bit mode, I finally got it not only to start up at all, but also for the SurroundPanner V5 to work even in 64-bit mode. So I'm still not sure what the source of the problem was, but very glad to have it working again -- I had been avoiding Nuendo and using Pro Tools (which I don't like as much but is stable) for surround projects since having this problem.
I've been confused about this for a while, which is that with Nuendo 6 (as opposed to previous versions) the SurroundPanner V5 no longer includes the graphical display of what's going on with the surround panning -- only the controls at the bottom of the window -- and the panning display in the MixConsole is now just a blank area. But I can't find any discussion of this online or in the forums. Could I be the only one experiencing this? What am I doing wrong to be missing these things?
I've been wanting to try the new Anymix, but when I follow the instructions in the Operation Manual, p. 297:
The Anymix Pro plug-in can be used as an alternative to the default surround panner, the SurroundPanner V5. To switch the panner, right-click the panner in the MixConsole and select “Anymix Pro”.
I find that the panner in the MixConsole does not react at all to a right-click or control-click. There is no way to switch the panner. The only way I can find to access Anymix is as an insert, but the virtue of Nuendo is having these things right in the channel.
What am I missing? I'm using Mac OS 10.8.5, and have been using Nuendo for years.