Dear Teerawut,
I am also very sorry that I couldn't talk to you at anytime. Here at work is no much internet because still under construction. Below hope will assist you.
The only mistake in your offer letter greeting Ms. D'Souza
If you receive the new offer letter sign it, and scan it, and send to Araceli, email araceli.tubo@sheraton.com
You need to visit a any hospital doing medical for people going abroad, any clinic or hospital approved by any GCC ( middle east country), just tell them you need medical for Oman and they will know what to do. After you send all the result with DHL or FedEX or TNT with address below;