Does "Ni zai nar? Ni zuo shenme?" correctly express "Where are you? What are you doing?"
We usually don't say "你做什么(ni zuo shen me)". We only say it when we are shocked, like when you just meet a man trying to robber you, you'll say that to him.

If you're asking someone what are you doing, you should use "你在做什么(ni zai zuo shen me)".