I just checked my Mac and I'm not experiencing this bug with a trackpad or mouse. As long as I hold down the Show Snapshot button it's there.
What kind of mouse are you using? Have you tried trashing your prefs? Just mash down on all the keys on the left side of the keyboard and click the AE app icon to open AE and you'll get the option to do so. Shift + Alt/Option + Cmnd/Ctrl + click to open the app
Thanks for the idea, but I'm sorry to say, it didn't work.
I use a Wacom trackpad and a Magic Mouse here in the office, nothing special. I checked at home and with the trial on my 2012 MacBook and everything works fine (I use an old pc mouse and the trackpad).
The problem appears also on my boss' 2011 MacBook, also AE CS6. And I found one guy in the CreativeCow forum with the same bug but no helpful hints as well. Thats all, I'm absolutely out of ideas.
OK, I found out the problem (and that I'm too easy to distract for problem analysis):
the show snapshot button works with the magic mouse, it doesn't work with my wacom tablet (I just tried the different settings, all with the same result).