https://www.fixtrading.org/orchestra-tutorials/ is divided into “Concepts” and “Hands On”. Concepts Part 1 should link to https://github.com/FIXTradingCommunity/fix-orchestra/wiki/Concepts-Part1-MessageStructures. In fact, it links to the Hands On tutorial at https://github.com/FIXTradingCommunity/fix-orchestra/wiki/Hands-on-Part1-Accessing-Message-Structures.
Thank you for raising the issue, should be resolved now!