As far as I understand, the mkgmap --index option produces "half-cooked" files that must be transferred by MapSource to the device. Once that is done, the search should work. Others can hopefully chime in; I have never used MapSource.
Same situation for me. I have never tried, but am in the opinion it should work this way. Can anyone confirm this? If so, it should be possible to find the differences in mkgmap and mapsource generated indexes.
I can confirm this: after transferring maps to my eTrex with either Mapsource on Windows/Wine or MapInstall on the Mac, address search works -- at least with the known limitations of mkgmap's index generation.
I have not recently attempted to copy a generated gmapsupp.img file to my device, but I recall that the address index did not work in this case. I assume that this has not changed.
I can confirm that copying gmapsupp.img to a Nuvi 255 results in not working adresses with a recent mkgmap. It cannot find any number in the street, even for streets which do have streetnumbers in them. Regards, Rutger.