If you're trying to override the machine's default application settings
(such as adding a custom extension for one particular IP or something), you
may need to click on the Create button first. It'll ask for a name, but you
can put anything in there (I don't even bother changing mine, but then again
I don't think I have any that override the defaults anyway).
For more information about this list, including removal,
see this url: http://www.iea-software.com/maillist.html