why is Macbeth so anxious to find the witches?

I am thinking you mean the second time he meets the witches just after the banquet scene with Banquo's ghost. Macbeth is in pretty deep into murdering people. He is King but he is paranoid that he will be found out. He also cannot tolerate the witches' prediction that Banquo's sons will be kings. Macbeth wants to find out more from the witches so he can know how to act. Of course, the witches will just mess with Macbeth even more: that's what witches do!