I am using Window Live Writer for blogger blog.It is a wonderful tool and it has lot of features. And When i started using this i could see there is a problem in split post option(Read More Option) and it is not at all enabled(Grayed out).Now i solved this problem with below techniques.
If you have a same problem following steps will help you to solve this problem.
1.Download the plug-in InsertBloggerExcerpt.dll from below location
Copy the plugin to this location
C:\Program Files\Windows Live Writer\Plugins
3. Close and open your windows live writer.
4.Check it is there