After installing the add-on, use the "Import All eml files from a directory" menu item to import all of your Windows Live mail folders.
Locate the folder containing your Windows Live Mail Folders for importing. For most users on Vista this is: C:\Users\<your username>\Local Settings\Microsoft\Windows Mail\Local Folders
The folders should get created in Postbox but it may take a little while for the add-on to import all of the messages into the folders depending on how many messages you have.