What is the purpose/function of the water pan?
What are some basic guidelines/rules for using it?
It helps to control the temperature inside your smoker. Jeff Phillips has a website that offers some free eCourses (5 total) that you can sign up for on the Web site: http://www.smoking-meat.com/ On the Web site, go the section that says "Free Stuff" and sign up for the eCourses. They provide you with a ton of information.
Your user manual for the MES should recommend how much water to add.
Hope this info helps you out.