I have 40 Grand Repair Kits. None of them I can use to repair my armor. I can sell, stack, unstack, get help, destroy, and put to text in game. I have no "use" option that will let me actually make use of this item.
Have you tried right-clicking on the armor piece to be repaired and checking the options? If it needs repair, you should have an option in the list to repair, just like you would for "Enchant" to change enchantments.