The part of the guide that talks about the MAC address is for setting up your router to assign a static IP to your computer. If you already have a static IP assigned to your machine, you can skip that step altogether.
(In any case, if you need to find out your MAC address you can always try this at the terminal:
macbookraf:~ raf$ ifconfig en0
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
ether ab:cd:ef:12:34:56
media: autoselect
where "ab:cd:ef:12:34:56" would be the MAC address.
Raf.