there should be a plastic nipple type thing at the bottom of the water trap that covers a screw type thing, take off the cover and loosen the screw till the water comes out (into a glass or something so you can dispose of it). The screw might be uncovered, in that case just loosen it.
If it doesn't have anything at the bottom to unscrew, you should be able just to uscrew the whole clear plastic container from the moisture trap top and go empty it...