To learn more about the current status of Textpattern’s support for MarsEdit, please see this thread on the Textpattern forums.
Select "Other MetaWeblog-compatible" from the Software popup in MarsEdit’s blog settings dialog.
After installing the XML-RPC package, the API Endpoint URL for your weblog should be: http://[YourBlogAddress]/api/
For more information, check out the Red Sweater Forums conversation about Textpattern: