Okay so alot of people have been having trouble on how to use multiverse Core so will tell you the easiest way to use it
Right so you have multiverse installed and you're wondering how to hell you use it so the masic commands are /mv this opnes all the commands you can do it will look like this
====[ Multiverse Help]====
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Add a '?; after a command to see more about it 
/mv [FILTER] [PAGE #]
/mv version -[pb]
/mv list
/mv info [WORLD] [PAGE]
/mv create { NAME } { ENV} -s [SEED} -g {GENERATORID]] -t [WORLDTYPE] [-n] [true:false]
[mv clone { TARGET ] { NAME } -g [GENERATOR:ID]]
you dont need to use all these commands to simple create a world do 
/mv create world normal this will create a normal world randomly generated
/mv create world normal -t flat will create a flat world for you
/mv create world nether will create a nether world for you randomly generated
/mv create world end will create a end world for you randomly generated
okay to actually import a world in you will have to you your world in the files for your minecraft server 
to import the world do 
/mv import [WORLDNAME] normal its as simple as that 
If you found this tutorial helpfull be sure to leave a diamond and a sub and as always if you want me to do more tutorials leave a comment in the section below and i will get to it as soon as possible