you need to place the new file in the folder where all wmaps are , then to visual studio, add existing item and select all files, find the new nexus file in the worlds folder u just put it in, go to propeties as guy aboved me said, change to the thing on propeties, then got to Nexus.cs and you will see realm/worlds/nexus.wmap/
rename new file u .wmap to ex. nexus2
and replace nexus.wmap with nexus.2wmap then rebuild have fun