This error when using Shockwave Player has been around since version 7 but the only advice from Adobe is to upgrade to a newer version of Player. Well I just downloaded version 11 and the error is still present with the following test site:
http://www.pbs.org/wgbh/mystery/murder/index.html

I'm using SeaMonkey 1.1.10 and the Adobe file sockwave_installer_slim.exe correctly installed the plug-in which is shown as active. I've gone to the Adobe test site and it indicates that the Shockwave plug-in was correctly installed. However, the test site given above always gives the "missing xtra" error.

On my WinXP system, there is the following directory: windows\system32\adobe\shockwave 11\xtras . The number of xtras actually in this directory are limited but it does contain the file, autodownload.txt . This text file contains the url for obtaining needed but missing xtras from the following site:
http://download.macromedia.com/pub/shockwave11/xtras
However this site does not seem to exist! All attempts to get to this site http://download.macromedia.com result in "The requested URL / was not found on this server."

This is a major bug with Shockwave. I've tried to report this to Adobe but Adobe does not seem to care unless you have purchased a product or a service contract. What can be done to correct this missing xtra problem?

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Right, somewhere in the many messages on this site I found an answer. The problem is several fold. Shockwave does not work correctly in part because parts of the program expects .x32 files to be installed in
Windows\sytem32\Adobe\Shockwave 11\Xtras while another part expects .x32 files in
Windows\sytem32\Macromed\Shockwave 10\Xtras . Further, by default the Slim version is supplied by Adobe not Full. Slim expects .x32 files to be auto-downloaded as needed but the download site is no-longer available.

Even if you install the Full package, the program won't work because the Macromed directory will be empty. Attempts to access Shockwave programs from web sites will result in some .x32 files being placed in the Macromed directory but not most. Instead the obsolete autodownload.txt file is installed with reference to non-existent urls
http://download.macromedia.com/pub/shockwave10.4
http://download.macromedia.com/pub/shockwave10.1

An Adobe white paper on Director 11, indicates that Shockwave has limited backwards compatibility. So this solution is problematic as a version 11 .x32 file must be used instead of a version 10 file. In some cases there may not even be an equivalent version 11 file. Nevertheless this specific error of the missing decompression extra is soved by copying swadcmpr.x32 from the system32\Adobe directory and placing it in the equivalent system32\Macromed directory.