1. Get the latest Multiverse version at http://dev.bukkit.org/server-mods/multiverse-core/
2. Set it up the usual way and make sure that you have the Nordic.jar in your plugin directory.
3. check if Nordic is loaded properly with /mv gens, Nordic should appear in the list.
4. if so, just use Multiverses create command like you used to but with the -g switch. If you would like to create the world mah_awesome_nordic_word with the seed ileikturtles you would do it like this:
/mv create mah_awesome_nordic_word normal -s ileikturtles -g Nordic
for more information on the command see the multiverse wiki: https:github.com/Multiverse/Multiverse-Core/wiki/Command-Reference#wiki-create
now you have a new world in multiverse using the nordic generator.
If you still have problems, check out the multiverse wiki page for custom wgens: https:github.com/Multiverse/Multiverse-Core/wiki/Custom-Generator-Plugins