When you use the old units module you have to insert a control space with “\ ”, e.g.
\usemodule[units]
\starttext
6000 \Unit \Kilo \Meter \ or 2000 \Unit \Kilo \Meter.
\stoptext
but when you use MkIV you can use the predefined \unit command:
\starttext
\unit{6000 kilometer} or \unit{2000 kilo meter}
\stoptext
Wolfgang