What is that mission? And how to trigger it?
DevBuilds are Developers Builds ...
Consider a DevBuild as an Alpha-Version or Closed Beta ... for a (very) limited circle of players ... mostly to test out (game) mechanics or plots ... to find and hone out bugs and glitches ...
DevBuilds are not ment for casual players ... they tend to have (new) bugs and are often unstable ... most certainly their save-files are incompatible with those from earlier and later versions ...