The 'Markdown' folder name is case sensitive' and it found at:
~/Library/Application Support/Tinderbox/Markdown/
The folder holds a file style.css which is used to help rendering the in-app preview of Markdown-based notes'. The CSS style can be user-edit to customise the otherwise generic HTML styling used in the text Preview mode.
To make it easier for the general Mac user to access Tinderbox's support folders, the Help menu's 'Reveal Support Folder in Finder' option will open the folder in a new finder window.