Just get the user who wants to retrieve mail to go to:
http://www.mydomain.com:20000Sorry I can't answer your question regarding the specificity of where to click what. I am using Virtualmin Pro, so wouldn't have a clue how the Virtualmin GPL (free) user interface looks like. From what I gather, Virtualmin Pro is much easier to use than Virtualmin GPL.
Virtualmin Pro is still under Early Adopter (EA) period so licence fee is cheap. Buy yourself a copy of VMpro while it is cheap. Joe said your licence will be extended by 1 year if you buy during the EA period.
By the way, I am not Virtualmin's chief indian for sales and marketing. I am only a happy customer.