As for multiverse support - I am actually running the server, and I guess it's the VoxelModPack Plugin that I needed (not VoxelPlugin, I couldn't find that anywhere) - which won't really work on my 1.8 server. Back to square one.
I did try running it anyway and adding a regions.xml file with my world hashes in it, but it still didn't work, and there were so many errors on starting up the server that I don't think it's even possible. I'm no programmer, just a mom trying to make her kids happy!
You will need help from whoever is running the server. See my reply to raxo2222 above. The alternative is a UI asking the player which world he is in each time he enters a new one. This would be a little intrusive IMHO, but the only option without server plugins to send info about the current world. The plugins mentioned are for the server. Nothing the client can do