lacerveza

hey guys, i am renting an mc server which i am running voltz on, but i really want plugins, mainly essentials for setspawn. i have put the jar file into the coremods server and restarted the server (i have done all this several times) then it just says in my mc client "cant reach server" and the only way to get voltz working again is to reset ftp changes. i am new to this so if some1 could help or offer an alternative i would be uber grateful