1)You put the .dm file into the mod folder in your dominions game. Then inside Dominions, you go to 'Preferences', then 'Mod Preferences', and then you click on the mod you want active to turn it on, in this case, Worthy Heroes 1.8
After that, when you make your pretender, it's set up to play under the mods you had active, when you create it.
2)As for how many turns, that depends on the admin, who usually sets it based on what people in the game want. Usually starting at 1 turn a day, then after around 30-40 turns, to 1 turn every 2 days, and possibly after 70 turns to 1 turn every 3 days. This is due to the turns taking longer and longer to do as the game involves more massive armies and many spells.
3)Yes, the map is needed as well. You'll want to download it from the attachment link from Llamabeast's post at the beginning of the thread. Then you unzip it, and place it inside the map folder in your dominions game.
4)Yes, you should see it.
You should see it after you click on Preferences, then Mod Preferences.