We had this error in our computer labs and found that only a very few computers
could successfully run the decompressor and none others could.

On the working computers, we found that the Shockwave installer copies the
necessary XTRA files into ~/Library/Application Support/Adobe/Shockwave Player
11/Xtras/download where they are available only to the user account that
installed the plugin.

Copied the download folder to /Library/Application Support/Adobe/Shockwave
Player 11/Xtras/download and changed ownership and permissions to match the
other items in this folder. Now any user of the computer can successfully run
sites that require the XTRAS.

Adobe: place these files at the system level, not the user level.