Basic question: I would like the Marine's weapon to do splash damage. I look at marine unit in the data editor and on the weapon element in the folder structure that appears under Marine but I cant find where to indicate the weapon does splash damage. (I saw a folder like element named Splash Damage that has check boxes to say if the splash damage affects firends, enemies, etc, but checking these does not appear to give the marine any splash damage)

Not so basic: Burst Ability: Once I know how to change the splash damage as a default characteristic of a unit's weapon, I would like to create a "Full Burst" ability with a cool down of 15 sec that grants Splash damage for the next 5 sec.

splash damage for normal attacks:
Modify "search - area" in the damage effect. Make sure that you modify the search filter, too. Else your Marine may could kill projectiles like rockets, too. Normally, you exclude: hidden, invincible, stasis, projectiles, dead.