well everynight, when they have all settled down, pick them up and sit them on the roost, they *should* stay there, if not try putting them on the roost again, do this every night when you lock them up (or when ever they are calm enought to pick up) it might take a week or a couple of weeks but after a while they'll just use the roost!
i have this problem at the moment! i got 20 new pullets and they are VERY stubborn and never want to roost on the roost at nigh, and every night i do what i've told you to do but they keep jumping off when im moving another pullet and go back to where they want to sleep! its very annoying, but the are slowly learning and about half of them have learnt to roost on the roost!
anyways good luck!!