When using Apple AirPort Express Wireless-N WiFi Base Station as an extender for a wireless network is it as easy as plug and play or are there complications that may require a knowledgable computer person.

Have .... iMac 3.6, i5, running10.7
Airport Express as wireless connection to iMac. Modem connected through ethernet cable to express.

Have little knowledge of this procedure so any help would be greatly appreciated.
JJ