I had this problem, here I copied and pasted my reply on a thread on PBN.
I found a very easy way of installing this o-ring incase any one else is having trouble. Tie a very thin string to the o-ring and drop the end of the string through the hole at the bottom of the stem, then pull the o-ring into the groove and adjust it with a ball end allen key. Then take an exacto knife and cut the string as short as possible. I did this and left a tiny bit of string on the o-ring, built the rest of the reg, aired it up. No problems at all.Still working 5 months later.
Thanks - I used a flathead screw driver and it worked