If I understand you correctly, I think the problem is that the prop does not go on top of those two pins, rather the prop sides rest against the pins.

Attached is a VERY rough drawing of how I remember it. Hope this helps.

I eventually dumped the original spinner for a Dubro 1/4-28 Spinner Prop Nut (avialable from Tower).

When you get it done, I guarantee you'll enjoy the Avistar. It's a great model.

Neal