I did it!!!!
Set "WordPress" as Software. "Drupal" can't grab categories.
XML-RPC endpoint file is "example.com/xmlrpc.php" (This is nothing special.)
In this way, in "Options", MarsEdit can get "filter" information from my Drupal installation. "Filters" are equivalent to "Formatting" in the WordPress world. Also, "Comments" work for both "Open" and "Closed.
Posting with some categories selected work as well.
Don't be confused with formatting for preview and formatting in the Options in editing a post.