Your problem is not clear, as to the "circle key". What do you mean by this? Most blade adapters have either a separate square key, or a key formed inside the crankshaft hole of the adapter. You must have a key that fits in both the crankshaft and the blade adaptor, at the same time. Did you get the correct adapter, or just find one lying about?

I finally found the name for the type key that's required. It's a Woodruff key. My thinking is that the flat side of the key should go into the adapter keyway and the rounded side into the shaft keyway, but I'm not sure.

Your thinking is correct- provided it is the right style key for the shaft...

Thanks, guys!!