Thanks for the tip on re-assembly.
Yeah, I think that was my problem. In my case the head had been sitting for a couple weeks, so the valvetrain was relatively dry which I think caused the keepers to bind up. The only thing I had lying around was WD-40 which didn't do anything in that regard.
Oh, and the adapter came with it. It was just like another plate/ring that you could put on top of the retainer on an EVO - something like the springs are smaller or something.
All in all, great product that will stay in my garage as long as I own a DSM or EVO.