The password feature is a temporarily solution, until Valve fixes sv_password for Left 4 Dead 2. As such, expect it to be "as good as I can make it".

To the point!

For players:

First enable your console.

See this picture for a short how to:

Then upon start of the game you can type the password into the console.

All passwords must be prefixed by sv_password and encapsulated in quotes. For example: sv_password "server password here".

For server operators:

Simply set the confogl_password cvar in the Confogl settings file to your password. For example, confogl_password "my password".