Need more info... When you say "ability" do you mean a racial ability or class ability? Or you trying to say "attribute score" like Strength?
If you have actually added a new attribute score (I think I recall you saying that years ago). Then getting it to level up also is going to be hard. Allot of such logic is "hard-coded" into HL to only deal with the 6 attributes.
Maybe if LW is really nice they could provide the raw XML code but without that I can't even guess.
I assume you need to add new Thing to this table:
AttributeIncrease.jpg