If you want SuperContainer to work in Web Direct use the Mac version (it works for both 32bit and 64bit server). Some people will say ise the Windows version but that wont work.
Install it in the following directory:
/Library/FileMaker Server/Web Publishing/publishing-engine/cwpc/Plugin
After that make sure to propagate the permissions from the Plugin folder, because if you just change the permission on the plugin file itself the package contents will not take it.
After this you need to stop/start your server. Easiest to do it from terminal:
sudo launchctl stop com.filemaker.fms
sudo launchctl start com.filemaker.fms
Then you should be good to go.
If you have a two-machine installation, the plug-in has to go on the Worker server (the web server).
This, obviously will work for other plug-ins you'd like to use in Web Direct.