on my coilovers, the spring perch is really two lock nuts. twist the top lock nut so it goes up the threaded rod (the vertical shaft of the strut/coilover/dampener), thus compressing the spring (adding preload). tighten the lower lock nut against the upper one to jam the spring perch (two lock nuts) into place.
not all coilovers are created equally. I suppose its possibly yours wont even allow you to adjust preload but that would be quite a pittiful coilover.
pics might help.