FT doesn't do atmospheric moisture and temperature transport. As a result, the results are a bit unrealistic (to put it mildly). http://www.ridgenet.net/~jslayton/climateinfo.gif shows FT's internal average annual temperature/rainfall to climate type table. You'll note that it doesn't include things like "mountains", "hills", or "plains" because those can't be determined from the simple temperature/rainfall table. FT really ought to have a "physical geography" visible layer with mountains, hills, plains, shallow ocean, and so on and a "biomes" visible layer much like the current "climate" one.

FT normally doesn't generate average annual rainfalls and temperatures that hits the "desert" spot on the table. FT really needs some toys for atmospheric transport (even just a latitude-based rainfall model would help) for good results. In the meantime, judicious use of the temperature and rainfall painting tools is your best bet for getting deserts to appear. You'll need to figure out where the deserts go and then (usually) lower rainfall until you get desert in those areas.