MultiMarkdown Composer has several features to enable you to easily navigate your documents.
The Table of Contents Panel allows you to easily navigate longer documents with your mouse/trackpad or with the keyboard. You can also quickly search your document to locate pertinent sections.
You can also use the “Move to Previous Section” and “Move to Next Section” commands (or the key combinations). These allow you to jump forwards and backwards through the sections of your document.
Additionally, all of the usual OS X system key commands to navigate text continue work as expected: