From the parts in the first pic...you're missing the tiny/tiny little spring that sits in the hole of the selector lever. The pin goes onto of the spring and, as said, clicks into each of the position holes on the body. Hopefully it hasn't dropped out and it's still in the selector lever.
If you've lost that spring...good luck fiding a replacement. However you can use a little bit of silicone wire insulation that's springy...but won't be the same as a spring. (a tiny dab of grease keeps the spring and pin in the lever while you're putting it together).