Those adapter tips from Radio Shack are meant to plug into Radio Shack AC adapters only. The AC adapter doesn't end in a tip, it ends in two holes, and you choose the correct size tip and plug it in one way or the other depending on whether the plug is supposed to be positive center or negative center. I've never seen an adapter that plugs onto an AC adapter tip to allow it to fit into a different receptacle, but I suppose it could exist.
I use that Radio Shack AC adapter that Necromancer linked on my PC Engine Duo and it works perfectly. It cost too much, but that's Radio Shack for you (my original NEC unit blew out, and I was desperate).