I'm need to know about the way the preferred bit works.

I see that a preferred location can sport an icon. Does it also sort the preferred listing to the top of the list. Say, there were 10 stores within 10 miles of my zip code. In that list of 10, three were preferred stores. Would those three list at the top of the list and be sorted in alphabetical order?

Second, when purchasing the product, I see it wants to create an account on the forum. Since I already have one, what should I put in that field?

Yes, that is exactly how the preferred locations are displayed. Always first.

The account you are creating during purchase is a Member's Account. That is not for the forums. You can choose any username you want.

The member's account is where you login with your username and password and download your files.