Click on + in the bottom left portion of the MarsEdit window to start the account manager.
Use the following settings:
Software: Other MetaWeblog-compatible
RPC URL: http://your.blog.com/(context)/xmlrpc/(blog-id)/
Blog ID: You will need to setup a separate Weblog for each category in your weblog. Use the category URL in this field. If you had a category called "test", this field would be set to "/test".