How do you ability units in kingdom hearts 358-2 days?

Answer 1

Two of the Ability Units can be obtained by filling the mission gauge 100% for missions 15 and 30.

Later on in the story you can buy a third Ability Unit From The Moogle Shop once you become a 'Master Rank'.

Answer 2

Place the ability units within the outlined shape created by the weapon you've placed inside your unlocked panels.