AIUI the citizens in civ5 can work up to three rings of hexes around the
city itself (36 tiles). A new city starts with only the first ring of
six hexes workable and has to expand its cultural borders out to include
the other rings. Beyond the radius of those three rings, the tiles are
out of that city's range, even if they are within your national borders.
If you click on an unemployed citizen and the game doesn't assign him to
a tile, it must mean that all your workable tiles are in use. Of course
there might be tiles that are within the three hex radius but aren't in
your cultural borders yet, so you might be able to expand out to them.
Apart from that, you can assign unemployed citizens to be specialists in
a library, bank or whatever.
If all the specialist slots are full as well, I guess you're stuck with
the unemployed citizens. They still give 1 production per turn, but cost
1 food and 1 happiness as well.
aaah, mahalo again.... - redvet, who is stuck....