I've been at this for hours, trying to find the file realmlist.wtf. I have used the search function, to no avail. I've been a computer geek since the 80s, but I feel like a total moron with this one.

realmlist.wtf has to be in the root folder of the game (world of warcraft\)
afaik it is only created when you join a server.
So just make your own by creating a new text document and rename it
set realmlist logon.retro-wow.com
set realmname "RetroWoW"