Finding the default gateway IP address of your router on your Mac is easy:
Open Terminal (don't be scared, it's easy). To open terminal, simply type terminal into spotlight (the magnifying glass on the top right of your screen).
With terminal open, type: "route -n get default" (without the quotes) and press enter. You will get a list showing amongst other things, your default gateway IP address of your router (which is probably: 192.168.1.1). Easy huh?