Hi Michelle,
Thanks for the pictures, I just left you a phone message...
so I just figured out that the middle piece that you have in your hands in Photo 3 is an extender it basically extends the neck of the collection cup for drier foam, so the piece in your hands that that we are trying to put on top of the skimmer, is where the collection cup goes.
Now if we try to remove that piece that is on the skimmer body by gently twisting it to the left with just a little force we should be able to remove that one piece, now look either on the actual piece we just removed or in the skimmer body where the grooves are there should be a O-ring. Now that same O-ring can also be used on the other piece that is in your hands in Photo 3. What I have found out just now that it is pretty much impossible to use both pieces at the same time.
Like I mentioned before If the o-ring is missing or you want a back up for those Just in case days, I can contact Jeremy at Aqua-medic and have them send you one.
Thanks again for your nice comments and your patience!
Walter