Start the game with the command-line parameter:

 -cmd:"gamegod.studiomode 1;sys.runcode studio"

 E.g.: C:\dreign2\dr2.exe -cmd:"gamegod.studiomode 1;sys.runcode studio"

 Then start teh game, click on TOOLS and choose load. Now select a campaign
 and mission. Once it loads, click on tools, then play mission.