This is Tim from KC101.com.
Thanks for visiting us here in the K-forum.
Okay, let's talk about your question...
You asked about,
"화장실이 어디에요? Is this formal? and How do you write Male and Female in Korean?"
Yes, this sentence is good to use in many occasions.
It's right between being formal and being informal. I, myself, would say that to anyone when I ever look for a washroom.
Here are tips.
If you want to make it 'real FORMAL', say, "화장실이 어디입니까? or 화장실이 어디에 있습니까?" (to the elder)
If you want to make it 'real INFORMAL', say, "화장실이 어디야?" (to the younger)
in between those two, you can say, "화장실이 어디에요?". That's why it is a perfect sentence to say!
Okay, "male" is 남자 / 남성 [nam-ja / nam-seong] in Korean.
"female" is 여자 / 여성 [yeo-ja / yeo-seong] in Korean.
If you want to say, "where is man's washroom?" then, you can say, "남자 화장실이 어디에요?"
If you want to say, "where is female's washroom?" then, you can say, "여자 화장실이 어디에요?"
well... that's all~~!
I hope my explanation helps you understand Korean better,
cheers,
Tim