I have uploaded a single-player world into my server files. (The World is called "Chamber"). I try to go to the Nether that belongs to Chamber but the Nether Portal does not lead anywhere. I know the way a server stores the nether is in a separate world folder to the world it belongs to (e.g. World and World_nether), however a SSP world stores the nether in a folder called DIM-1 which is inside the main world folder. How can I link the nether portal in Chamber to the nether that it should be connected to? I have tried /mvnp link nether Chamber Chamber_nether, but it tells me that Chamber_nether does not exist.

As far as I know, CraftBukkit should convert the singleplayer-world to a multiplayer-world (with world_nether) when you load it. But I think MV won't automatically import it so it tells you that the nether doesn't exist when you try to link it with MVNP.

You need to copy the files from inside DIM-1 to a new folder in the server directory called Chamber_nether. And then of course import it like usual.