Freighters act passively. Once they're built, they just sit in a pool until they're needed. As soon as there is a planet with a food deficet, and annother with a surplus, one of your freighters will become active in order to balance the problem. The number of free and total freighters you have is recorded in one of the squares on the far right side of the main screen.
As an aside, moving colonists requires the use of 5 freighters for each unit of population.