/rcon login [your rcon pass]
You can get your rcon pass from server.cfg
Example
Open server.cfg using notepad.
[Server.cfg]
rcon_password lordztest
port 7777
hostname D Generation X Fun Time Server
So,here I've made my rcon pass as lordztest
/rcon login lordztest
Then they will ask for Second Rcon.
Type changeme in second rcon and login.
Then Use /setlevel to set your level.
But while starting server,dont forget to change the rcon pass.