Sorry, I never visit other forums than the 'Development' one.
If you've installed KompoZer from the get-deb repository, it's impossible to install an extension unless you run it as root. After some tests, all I can say is that I strongly recommend NOT TO USE this version of KompoZer.
This will work on Feisty. Versprochen.
I'll update the kompozer.net website ASAP.