(1) if you have never been to china, you'd better say:我不在中国。
(2)if you have been in china, and someone knows this. but you aren't in china when you are talking with the one, then, you'd better say:我没在中国。in this case , you can also say:我不在中国。
(3)the structure:
我没在中国。
I am not in china.
subject: I 我
link verb: am (in this sentence, chinese do not have is part)
adverb: not 没/不
prepositional phrase: in china 在中国
PS:"没",not only means "have not", also can simple mean "not".