Most of the files in the campaign layers (Data/Campaigns/Campaign) are "random layers." That means you should launch the editor and "Open Rnd Layer" then select one. They are the guys with names like:
41a_Jap_Convoys.mis
41a_Jap_SubHunters.mis
The missions with "BattleofXXXX" are "scripted" layers.
Anyway, if you open them up and save them the wrong way, wierd things happen. I saved Campaign_LOC.mis the wrong way and made it disappear, lol. If you are unsure, do what I did, make a copy of the entire Campaign folder someplace safe
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Anyway, you can right click on the random groups (yellow with a ? on them) and select group properties for stuff liek speed, chance of occurence, etc. Alternately, you can select "Define Group Contents" which will give you the ability to edit what kinds of ships are in that group, their skill level, cargo type, etc.
If you right click in the middle of no place, you can add a new random group. To add traffic someplace that's what you'd do.