You need to use
/opt/sigil/sigil.sh, this script modifies the LD_LIBRARY_PATH so that the Qt libs bundled with Sigil can be loaded.
FWIW, the error you got with the Ubuntu package is probably due to your system defaulting to use the GTK+ Qt style, and it's complaining that it can't detect your current GTK style. Anyway, you can workaround that error by specifying the Qt style, e.g.:
Code:
$ sigil -style Cleanlooks
$ sigil -style Plastique