Did you install the homescreen loader?

"Desktop widgets require a two step approach. First, a special loader is required in order to load Qt based home screen widgets. This loader only has to be installed once, and usually, the Debian packaging system should be used to make sure it is installed.

In your home screen widget's Debian package, make sure you depend on the package qt4-homescreen-loader. An example "Depends" line could look like:

Depends: libqt4-core (>= 4.6.1), libqt4-gui (>= 4.6.1), qt4-homescreen-loader"