How can I use Airdrop in my iMac which has OX 10.9.2

You need another Mac in close proximity which also supports it.

Does yours support it? If so, it will automatically show in the sidebar of a Finder window.

http://support.apple.com/kb/HT4783

Unfortunately not. Airdrop is only supported between Mac OS > Mac OS and iOS > iOS.

If you have an iPhone5 and an IMac,

- you should have your photos in phototstream (last 1000)

- your iphone should have your icloud account linked

- your imac should have your icloud account linked

more info http://support.apple.com/kb/HT4486

https://discussions.apple.com/thread/3903244?tstart=0