To think you've pushed that pin all the way through the arm and the lever when you haven't really done so, and in that case the C-clip doesn't have a place to fit. Well, actually it DOESN'T have a place to fit if the pin isn't pushed all the way through the lever. It's easy for the clip slot in the pin to catch on the lip of the hole in the bottom of the lever without your realizing that's what happened.
The simple solution if that's your problem is to use some 'wiggle and jiggle' action on the lever while simultaneously pushing down against the pin. Use a little muscle while you're doing that, although it shouldn't take too much. In any case, most of the time what you'll find is the same thing that I found--the pin is fine and just wasn't seated 100% of the way through. Do what I described and you'll likely find the pin drops nicely in place, as it happened to me on many occasions while tuning other people's rifles.
Oh, yeah, another thing that can make it fairly impossible to get the pin all of the way through: If you tightened that silver hex nut on the lever such that the lever doesn't sit perfectly square inside the cocking arm, the 'off-angle' that's created pushes the clip end of the pin away from the hole in the cocking arm, and the pin won't go all the way through the cocking arm. The size of the hole in the cocking lever is VERY close to the OD of the pin, so if you don't have them aligned perfectly the pin won't go all the way through. What do you do to see if that's the problem? Loosen the nut, turn the rod a tiny bit, and try again. Once the pin drops through, re-tighten the hex nut with the lever in that position. If you prefer, however, you can use a pin punch to align the parts pretty closely before you even try inserting the pin.
I never saw even one single case here where the end of the pin had actually been sheered off, but I had a bunch of cases where the pin wouldn't go through both parts until I fiddled around with all three pieces as described above. I'm not saying it can't happen, I'm just saying that I never saw it here.
There's something else you don't seem to mention here: Did you simply try pulling the pin back out and looking to see if the C-clip slot is still there on the end of the pin? If the slot is there, you obviously didn't have the pin pushed all the way through. Try the above tips and, voila, it should go--success!
Safe and Happy Shooting!
Ed Krzynowek, The Airgun Tune-Meister
"We can rebuild the squirrel. Make him stronger, faster...We have the technology"---Skyler M.