I've got some old aerobars from profile design. If yours are anything like mine, the "shims" are metal. and 2 different shapes. For a 26mm handlebar, you stick one shim above the bar and one below it. Then the clamp should fit. In other words, 2 shims per side. Also each shim should have a little raised bump. That bump matches an indentation in the clamp to keep them from sliding after you tighten them.

I hope that makes sense. I'm too lazy to take a picture of my bike.