Thanks, Peter. Not what I wanted to hear, but it appears to be right on point.
To summarize, the Canon XA20's .mp4 files do not work natively in CS6 but do work in CC.
I purchased the CS6 Master Suite four months ago when I ordered my camera because .mp4 files were listed among the supported media. Unfortunately, I can't realistically upgrade right now to CC after dropping big bucks on the suite.
One of the reasons I'm shooting mp4 is that I've discovered there's also a problem with spanned clips in AVCHD - when they get past 4GB. So, I'm kind of boxed in.
Nonetheless, Peter, I much appreciate your prompt response and any suggestions you might have on my options.
If your clips are less than a minute in duration, they will not be affected by the spanned clips bug. Files generally need to go over 20 minutes to span.