You do add your Strength Mod to the Damage. On page 145 of the core book under damage. Under strength modifier near the bottom of the damage section it talks about light weapons. It is actually referring to page 120 under Weapon Sizes where there it talks about light weapons. Light weapons are a size smaller than your size. So a looking on the weapon chart you can see what weapons have what size. What the Strength Modifier paragraph is talking about is that when you attack with two hands you double your bonus but not with light weapons. Meaning that if I attack two handed with a short lightsaber I only add my regular bonus, that is if I'm a medium size creature. Because a short lightsaber is a size smaller than I am. Lightsabers are Energy and Slashing Type weapons as per the chart on page 122. So with a regular lightsaber using it two handed you can add double your strength bonus to it if you're medium size. The lightsaber in the legacy book foregoes the extra bonus and raise your dice type instead.
At least that's what I think is the problem. I'm new to the game as well but I can't find anything else.
