The elastic cord loops (keepers) are probably the other part of the ice ax attachment; they looked like velcro to me in the other pictures you posted.

It didn't really look like you had any places on the pack to strap the pad on vertically, though that would be a good option with closed-cell pads. Under the lid should work, especially if the lid "floats" and can be adjusted to leave some room there if the main bag is really full.