well, my female took to it straight away!
i guess ur bun is indoors... put her/him in the cage, hutch ect and corden a bit off(like only the top bit of hutch). but it still has food and water ect... put some soiled bedding into the tray/litterbox. she should ctch on how to us it. give ger a little bit more room. but if sshe makes more than a "few"(5) mistakes in one ay put her in the small bit. do dis till she is trinned. if she is not netered/spayed she might stop it when he/she is "growing up" so get he/she netered/ spayed and she might do it again or a little trainnng again.
(dont keep ur bunny in tere all day let him/her out to play) he/she will soon use it.
(rabbits are clean, they uuse the same spot to go to the toilet, so put it in that place. resarch on wht to put n it. i use wood shavings)
hope it helps