You might be confusing RB3D with the original Red Baron which at some point did have an official "Mission Builder Expansion Disk"
There are some un official utility tools for RB3D though.
One is Capt Darwin's Full Covered Jacket Control Panel
Lets you tweak graphics, screen resolution and types of missions. Works with early versions of Win7, but some of the official MS updates have busted that.
Two is Tom's Campaign Manger
v1.1 or CM11. Has great features on dealing with one's pilot and helps with using date based patches. But does not work with Win7 at all.
Third is Editeur de Campagne pour RedBaron3D
which is in French. Very similar to CM11 in some respects, and very different in others. It is more squadron based then pilot based. Though it's date change ability is fataly flawed, it makes the date the current year you live in, not the year you are playing in (2011 instead of 1917). However it does work in Win7.
I have a copy of Editeur_de_campagne_pour_RedBaron3D on this page: https://skydrive.live.com/?cid=e196374a237958b4&sa=30411298#cid=E196374A237958B4&id=E196374A237958B4!120
I would check SWWISA for the other two, or i guess I could upload the ones I have if you can't find them.