it took me a little while to figure out, but be sure to set your RCON password in the server.cfg file. once you start the game client, you can open the console by hitting the "`/~" key. once you get the console open, connect to your server by typing "connect :27015" (unless you specified a different port for the server to use). once you are connected to the server, bring up the console again and type "rcon_password (YOURPASSWORD)" then type "rcon changelevel ge_(LEVELNAME)" at that point the level should change to whatever level you chose. if you skip the "rcon" the console will auto fill the different level names, so you don't need to know them, then just go back to the beginning of the line and type rcon. after the new level loads, open the console back up and type "rcon heartbeat" and after that, the server should show in the server list.
hope this helps...