Apparently the plugin was deleted by jDowloader team because the extmatrix.com domain is up for sale according to this bugtracker entry....
https://svn.jdownloader.org/issues/87096
This is confirmed with the extmatrix.class file being removed from the latest version Jdownloader. I tried adding the extmatrix.class file from backup but it doesn't work still.
Extmatrix seems to be still operating for me.