If you're a long-time Blogger user, there's a good chance you need to update your blog template to reflect recent changes Blogger made to their remote-editor interface. This page will walk you through the required steps to achieve this.
The first thing you need to do is log in to your Blogger account, and click the Layout button for the blog you are fixing.
The fixes we need to make are in the HTML code that makes up your template, so click the "Edit HTML" link from the top of the page.
You'll see a text editor with a bunch of HTML code in it. We're looking for a cluster of lines near the top that start with <link> and <meta>. We're going to replace these with a more modern equivalent, so select them and delete them from your template. Don't delete anything else, though. It might be important!
Everything we deleted above is going to be replaced by a simple tag that lets Blogger essentially generate all the correct meta information for your blog. Just paste the tag "<$BlogMetaData$>" into the template where the link and meta items used to be.
You're done fixing the template! Now you've just gotta save the changes.
Your blog is now "fixed" so it will work with MarsEdit. You can either start over by adding a new blog in MarsEdit, or open your existing blog configuration and "re-detect" the settings.
When you detect the settings, you should see some obvious changes. The RPC URL should no longer be "http://www.blogger.com/api", and your blog's numeric ID should appear in the blog ID field. Close the settings window, and you're ready to "Refresh" your blog to pick up the existing content.
If you're still having trouble feel free to contact us or ask in the forums.