Please click "Tools"->"Preferences" menu item in Extension Manager, check "Enable detailed logging" option. Restart Extension Manager. Try to install the extension. After the error message pops up, quit Extension Manager. Send the log files under "C:\Users\<YourUserName>\AppData\Roaming\Adobe\Extension Manager CS6\Log" (Windows) or "/Users/<YourUserName>/Library/Application Support/Adobe/Extension Manager CS6/Log" (Mac) folder to email@example.com.
Note that the "C:\Users\<YourUserName>\AppData" and "/Users/<YourUserName>/Library" folder is hidden by default. On Windows, you have to check "Organize"->"Folder and search options"->"View"->"Show hidden files, folders, and drives". On Mac, hold down "option" key and select "Go"->"Library" menu item to go to "/Users/<YourUserName>/Library" folder.
Thanks Carl for solving my Extension Manager CS6 problem with the following advice - after you'd sighted a copy of my ExtensionManager.log file:
"Your Windows account name contains two space characters. I am not sure if it's the root cause. You can edit TEMP and TMP environment variables to change your temp directory to "C:\Temp", restart your computer then retry."
I suspect the problem was solved by changing the environment variables to C:\Temp. I'm now able to install DW6 Extensions into the Manager.