For a first look, i must say - impressive.
Even if i just pressed some random buttons, since i haven't any experience in Mapper.
To save your time, i suggest you launch the mapper right in the repository folder, because it doesn't work in client folder, nevertheless what the guide suggest.
The map created with Tile Mapper should be placed into fonline_sdk/server/maps to be loaded. (Also, everything gets saved there)
Handy hotkeys:
Exit mapper: Shift+Esc.
Load:~filename(without extension)
Also, preset_cave.fomap is really worth a look in Mapper. And a little edit, too.