Is there a fan shaped Airport icon on the top rightish menu, if so click on it.
System Preferences>Network>Show:>Network Port Configurations and Airport checked ON?
System Preferences>Network, click on the little gear at the bottom next to the + & - icons, (unlock lock first if locked), choose Set Service Order.
The interface that connects to the Internet should be dragged to the top of the list.
At the Apple Icon at top left>About this Mac, then click on More Info, Network... does it show it has an Airport card?