If you'd like, I can find where to the latest version, or help you troubleshoot why your previews might not be appearing. PreviewMarkdown - App Store - Apple
Users can adjust the base body font, monospaced code font, and the colors of headlines and links via the app's Preferences panel. Installation & Setup PreviewMarkdown 1.4.5
Press Space on a Markdown file in Finder to view a rendered version instead of raw text. If you'd like, I can find where to