The applet should be installed with the usual ./configure & make & make install sequence.
Then you should open up Kicker's panel/applet menu and select KLassie. The configuration dialog should come up. You can now pick either the IMAP or Shell providers.
If you have entered the connection information in the KDE module for email settings, in the control panel, the information will be copied.
You must provide the username, password and servername. The port should be a sane default, but it can be changed if your server is non-standard.
If you select the Stay Connected option, the provider will not close the connection between requests.
You should enter a shell command, one example might be "echo 5". This will just display the number 5, not very interesting, but a good test. This should execute the commands with your default shell, so any constructs from that shell should work. This uses the KShellProcess class, so any behavior can be described in the KDE documentation for that class. Any text after the initial number will be displayed in a tooltip.
This allows you to enter a url. The number of files in the directory will be displayed. This allows the use of any KDE kio slave. The best examples of URLs are imap://localhost/INBOX?UNSEEN and pop3://localhost/.
Once running, the provider will be polled for new counts at the frequency specified in the configuration.