for.
He was mad at her for not telling him the truth!
他对她发脾气了,因为她没有跟他讲实话!
第一,be mad at sb对...发脾气 tell sb the truth 告诉...真相
第二,根据句意,介词for在此表原因.
第三,特别注意介词后的动名词,其否定形式是在动名词前直接加not.个人认为这可能是困扰你的原因之一.类似的例句 His boss was mad at her for his being late again.他的老板因为他的再次迟到而发脾气了.