Here are the frequently asked questions. Please go through them before asking.
Q: Help, when I run the server I get this error:
[13:22:27 INFO]: [WinterSlash] null
[13:22:27 ERROR]: Error occurred while enabling WinterSlash v1.0 (Is it up to date?)
A: This is because the plugin wants to load the world but it doesn't know what the world name is, once you set the red spawn and reload it will work.