I have read and tried to figure out for myself, but must be missing something obvious...

I IWPed using FM Pro 10 Trial, which I was assured had no limitations. I can access via browser by ISP from originating computer, as well as from non-originating laptop (granted, they both use the same wireless internet connection), but my test person trying to access from their computer, can't access at all.

Can someone shove me in the right direction to fix?

Assuming that they're on a network other than your wireless network, you'll have to forward a port to the computer hosting the solution and then have your person access IWP on your external address... Just watch this: http://www.filemakerdesign.com/blog/?bid=11

Thank you! I did get it working... but ended up opting for hosting, which was a far better approach. Again, many thanks.