Default World Generator is a mod designed for Minecraft ModPack makers who want to have a specific world generator selected by default. The user is still able to change it if they desire, but it will change it from “default” to what ever you specify in the config file. This is a client only side mod, and does not go on the server.
Install the mod into your mods folder, make sure the mod that provides your world generator is also installed. Launch minecraft, once it is loaded close minecraft and edit the defaultworldgenerator.cfg file that is in your config folder. If you know the name of the generator, just replace default with the name.