I am probably missing a dependency of the scripts.
That appears to be my problem. After "apt-get install libxml-xql-perl"
the Dropshadow effect now works. The other effects (grid, blur edges,
etc.) still seem to have no effect. And since they are plugins and not
scripts, I really have no idea how to debug them. Any ideas?
Now that I have a working script to look at I should be able to try my
hand at writing some. Thanks for helping me stumble through this.
Hopefully I'll be able to help next time the question comes up.