I tried in frsutration many times to get this working using the latest stable and CVS package off the Webcalendar website. But found their being an error in the HTML web frontend installer; it would not let me set the DB type. So I tried to do it the manual way only to have a similiar error "DB type variable not set".
So realising Debian has a huge repo, I apt-get installed webcalender and then did a dpkg -L on the package to find it was installed to /usr/share/webcalendar/ and the config in /etc/webcalendar. So I symlinked to /usr/share/webcalendar/ from a folder on my webserver and off we went, no problems.
Seems so simply now.