Plugin is unrestricted mod which means that it can do everything since it's not limited by ModAPI provided by KSH. This has obviously it's pros and cons.
Having seamless plugin installed is not required to play. You can still play without it, but anytime you will change sector you will need to wait for your game to fully reload between servers, which can take few minutes. Seamless plugin reduces this to mere few seconds.
Follow instructions on Plugin Loader. Once you get that up and running simply go to Plugins in main menu of the game and add Nexus Seamless.