AirDrop was intoduced in OS X Lion and it’s an amazing feature! AirDrop lets you send and receive files between other Mac users easy and fast, even without being connected on the same wireless network! When I upgraded my OS to Lion, this was one of the first features that I wanted to test but wasn’t able to even see the icon on my Finder. Seems that unless you have a quite new computer, you’re not able to use AirDrop because your Wi-Fi card does not support PAN (Personal Area Networking). PAN enables your device to scan for other AirDrop users even if they are on another wireless network or not connected at all.
Being frustrated about not finding a solution, I went off to my corner to cry as a fanboy that lost his favorite toy but thankfully I’ve read on MacWorld that you can enable AirDrop on your computer even if your device isn’t natively supporting it!
All you have to do is to is to open a terminal window and type:
defaults write com.apple.NetworkBrowser BrowseAllInterfaces 1
After that, restart Finder and you’re all set! You still can’t access users if you are not connected on the same network but still, most of the times you want to share a file with someone, you’re on the same network.
Notice: You have to enable this on all your computers, even the ones with new Wi-Fi cards in order for them to start searching the network they are connected to. One drawback is that the user who has a new Wi-Fi card will lose the AirDrop functionality that can locate other users on other networks but hey, you can’t have it all! 
One solution that is not that irritating is to enable the interface to send and receive some files and then close it again. Since terminal has command history (pressing the up arrow key) you can easily do that in less than 20 seconds.
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