Indeed in Sub Command there is no box to make a campaign in the mission editor. That function only exists in Dangerous Water's editor. HOWEVER, you can still make a campaign in Sub Command, just it's a little more complicated. Here's a link to a tutorial on how to do it from
www.subguru.com:
http://www.subguru.com/Campaign.zip
Also, check out that site. There's a LOT of interesting tutorials and missions there that you can study and learn how to really spruce up your missions.