http://msdn2.microsoft.com/en-us/library/bb259698.aspx
Once you’ve enabled this feature (it may take a day or two to fully change over on spaces.com), you can add your weblog to MarsEdit and it will automatically detect the settings.
If the settings are not detected automatically, this is how to set them manually:
Select "Other MetaWeblog-compatible" from the software popup in the weblog settings dialog.
The API Endpoint URL for Spaces blogs is: https://storage.msn.com/storageservice/MetaWeblog.rpc