I had already read the garmnuvi.doc, and followed the procedure, but that seems to describe what to do with the official purchased file, not the "test" 3islands file. I did everything the garmnuvi.doc suggested, except for changing the name (which seemed to relate to the full operational file). There is also a 'read me' file which downloads along with the zipped 3islands test file. That describes a slightly different procedure, which also did not work.
The person answering the phone at BIZER said he'd never heard of it not working on any particular Garmin unit, so I'm still hopeful. Maybe I'll call him again.