Don't look in the \Appdata\Roaming\ftblauncher folder. Look in the folder where your launcher is (or wherever you selected for the "install folder" on the options tab), and you should see a folder with the name of the modpack you're playing. Open the "minecraft" folder.
If the world you're using for your server is singleplayer, open the saves folder and copy the world you want.(i.e. /Direwolf20/minecraft/saves/<world name>)
(Could someone clarify the process for if the world he's using is multiplayer?)
Regardless of whether the world that you want to use for your new server is singleplayer or multiplayer, delete everything in the world folder for your new server and paste the world into there.