If you go to the "In Battle" tab you can click on your other partners that are currently in your formation.
You have to complete the requirement which is shown when you hover with your mouse over the "Dam" icon.
After completing that requirement you can go to the "Attribute Bonus" tab where you have to put the required partner in the "+" slot in order for the skill to activate.
Upon completing the requirement (Dam icon) the partners will light up in the attribute bonus menu.
After aiding a partner (putting a character in the "+" slot) the skill is activated and the picture with the partner has an orange border.
I've got the same problem I encounter with conquest of might. The descriptions are all off to the side and I can't read them so I have absolutely no idea what to do.