Find the file you mean to edit

  1. Find the .md file in Finder. Check its name and folder.
  2. In MarkThatDown, choose File > Open File or press Command-O.
  3. Select the file. Check the title and contents before changing it.

When the file will not open

Check that Finder can access the file and that a synced copy is downloaded. Reopen the containing folder if permissions changed.

Opening a file and choosing its default app

You can make MarkThatDown the default Markdown app, but doing so is a separate choice.