This widget listens on DBus for Log events from XMonad and displays the formatted status string. To log to this widget using the excellent dbus-core library, use code like the following:

 import DBus.Client.Simple
 main = do
   session <- connectSession
   emit session "/org/xmonad/Log" "org.xmonad.Log" "Update" [toVariant "msg"]

There is a more complete example of xmonad integration in the top-level module.