If you press the "Refresh" button in the settings area (see second
screenshot here: http://code.google.com/p/tiecal/wiki/ScreenShots) you
should see a list of the available databases.
From the dropdown, select the one that is called: mail\yourname.nsf
(It's not always called that, but the usual setup is first letter of
first name, then the entire last name. So I have "mail\isavo.nsf" for
instance)
-Isak