If you are attacking with Polearm Master or Double Bladed Scimitar as a bonus action, do you get to add your ability score modifier?

With two weapon fighting, you need the two weapon fighting style to add the ability score to the bonus action attack, but polearm master doesn't say if the ability score is added or not.

It does actually say for Polearm Master. The second sentence describing the feat's details states:

"This attack uses the same ability modifier as the primary attack."

Double Blades Scimitar and the Revenant Blade feat don't say what to do, however. I would go with using the bonus to hit and damage as it doesn't specifically say not to. I notice the Great Weapon Master feat also does not specify either but, taking away to hit and damage bonus modifiers would really limit the strength of these feats. 

I didn't catch that, good on you. Thank you.

The general rule for weapon attacks is that you add your ability modifier to the damage. The only reason Two Weapon Fighting does not is because it specifically says it doesn't - that specific rule is required to override the general rule. So when it doesn't say either way, you can safely assume the general rule applies.