Hi there. Your blog's theme seems to have omitted the autoconfigure information, or else you are running a security plugin that omits the information from the blog's home page.
You can either manually configure it, by guessing the API Endpoint URL and inserting it into the MarsEdit blog settings by hand, or you can try switching the theme temporarily to a default theme, or disabling plugins before auto-configuring in MarsEdit.
If you try to set it up manually, the API Endpoint URL seems to be: