My six year old step daughter does not like our newborn and I have even seen her hit her. how do I tell my husband about it?

It depends on who the child confides in. If she has been on closer terms with your husband all the while then you should highlight this issue and let your husband talk to her. Since you all are a family now, I believe in practicing transparency. You should solve problems as a family. It could be she's feeling threatened by the presence of the baby or it may even be external factors leading to her actions. She may be experiencing some unhappiness in school but choose to take it out on a baby who can't complain about her.