Its extremely easy to use and requires no changes to anything.
Available at
http://www.swwisa.net/resources/rb3d.html The following two posts, extracted from Delphi, are probably a good deal more detailed than you need.
From Steve Fabert:
MissionGen works on any mission file that is still in its unaltered state as generated by the game, whatever patch was loaded at the time the mission was created. You can go back and apply it to missions that were generated long ago, if nothing has yet been done to mess with the mission. You must manually apply the program to the mission file while you are outside the campaign mode.
The sequence I use is to
1) Use the Next Mission button to generate a new campaign mission;
2) Look at the briefing page that shows waypoints and altitudes to confirm the game's original height assignments;
3) Exit the game and use Mission Gen IV to massage the newly created mission;
4) Reenter the game, restart the Campaign, and again check the Next Mission briefing page to confirm that the altitudes have been raised;
5) Play the mission.
My followup:
Pretty much what Steve said.
1. MissionGen only works on the mission file, not th campaign file. No need to delete anything.
2. Since MissionGen only works on the mission file, it will work with any UOP.
3. New mission files (campgnxx.mis) are created for every mission. To use MissionGen you have to:
Create the next mission (in RB Press Campaign - Fly Next Mission)
Exit RB
Run MissionGen and massage the mission file
Re-enter RB and fly.
Setup instructions are included in the Readme
There are three files that make up MissionGen:
MissionGen.exe - the executable. Can be anywhere but I put it in the campaign folder.
MissionGen.ini - initialization file colocated with the exe file.
Contains three lines:
path to campaign directory
path to simpatch directory
last user settings.
This file is preset for a standard RB install. If you have installed RBII or have RB3D in a directory other than the default, you will have to edit the first two lines
MissionGen.txt - data file located in simpatch directory.
Contains data used by MissionGen to calculate altitudes and mission types.
Edit this file if you want but be careful not to change the format.