'Now that' would be correct if something has changed. For example, the person you are talking about moved and you don't know where they currently live. 'I used to know where he lived. Now that I don't know where his house is, I can't go there.' If that is not the case, then I believe 'since' would be a better option. 'Since I don't know where his house is, I can't go there.' Hope that helps, Andrew