Select your shortcut to Dark Reign 2 and right-click on it.  Select 'Properties'.   In the section marked 'Target', add::

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

For example, if you installed the game in "dreign2" on your C drive, then enter: C:\dreign2\dr2.exe -cmd:"gamegod.studiomode 1;sys.runcode studio"

Start the game and you will see a 'Tools' button at the lower right of the screen.  Selecting this button will allow you to load any campaign that you want.  Click 'Tools' again and then you can chose to 'Play Mission'.