To tractor something, select your tractor beam (press g or G), point at the cargo, and press and hold the spacebar. Cargo must be within 1000m to tractor, and you must have space in your hold.
if you havn't bought a turret and just and a tractor, select thru your weapons with 'g' until you have selected tractor then press space, or if you have bought a turret select your turret by pressing ']' and run thru the same process by pressing g and then press space to activate the tractor beam.
I think that is right. Hopefully this is of some help