Bonus Ability scores

[ErinRigh]

I am trying to code an ability (or many abilities) that allows a class to add a bonus to an attribute at certain levels. Similar to the 10th level Animal Lord Ability "Third Totem". I can get the class to add the bonus with

hero.child[aCHA].field[aNormMod].value = hero.child[aCHA].field[aNormMod].value + 2

what I can't figure out is how to add the bonus at different levels of the class; say Charisma at level 1, Constitution at level 3, Strength at level 5, etcetera.

I can either get all or nothing. Either it adds all the bonuses at 1st level or I can't get it to add them at all.

Any help would be TONS appreciated

Erin