Is one AirPort Extreme already set up, working OK and you want to add another?
or
Are both AirPort Extremes not set up?
If both AirPort Extremes are not yet set up, we need the make and model number of your modem.
What model numbers of AirPort Extreme do you have?
FWIW, the support document that you reference is 3+ years out of date, so it would not relate at all to the newer operating system that you are using on your Mac.