This seems ridiculous, but I've read that taping a picture of clowns in an anemone to the side of your tank sort of gives them some inspiration: http://www.nano-reef.com/forums/index.php?showtopic=243505
It can take days or even months, but they'll find it eventually. Imo it's not worth stressing the clowns or yourslf trying to rush things. Feed them near the nem and be patient.
I got a pair of captive bred black clowns 1/2" and 3/4" for a month now and they haven't take notice of my RBT in my BC29. It took my pair of Ocellaris only a week in my 72. I would just wait it out.
Must be a case by case basis. My captive bred black clowns had no prob with it.....just a couple of days. Before they were in my nano and had started hosting in some fuzzy mushrooms.