There is a little plastic U-shaped slider that helps contain the trackball assembly (I'm assuming that you still have it in the white square plastic piece with the rollers still attached). Use a needle or paperclip to push this down towards the keyboard. It doesn't move far, just enough. Put the trackball in the slot with the "wings" out to the side. In the slot that the ring snaps into, there is part of that U piece, you can slide it up with that and then when you snap the ring on, it will all be locked in place.
There is a little plastic U-shaped slider that helps contain the trackball assembly (I'm assuming that you still have it in the white square plastic piece with the rollers still attached). Use a needle or paperclip to push this down towards the keyboard. It doesn't move far, just enough. Put the trackball in the slot with the "wings" out to the side. In the slot that the ring snaps into, there is part of that U piece, you can slide it up with that and then when you snap the ring on, it will all be locked in place.
I've taken apart many, many trackballs and have no idea what this piece you are referring to is.
