AIRPLAY: I cannot mirror my MacBook Pro (Early 2011) with an Apple TV 3rd gen and the latest software.

Hi,

I have an Macbook Pro (Early 2011) with Mac OS X Lion 10.7.5. The software is updated and I am using a Cisco dpc2325 router provided by my ISP (Cablevision/Fibertel from Argentina).-

I have been trying to mirror my screen with my apple TV and the aiplay icon never shows up on my screen. Nevertheless, when I run Itunes (also updated) the airplay icon appears and, when activated, my library (Music and Video) can be accessed through the TV, but no the actual screen of my computer.-

I don't believe there is a problem with the router's configuration as I also have an IPAD and my ipad gets mirrored with the TV without any inconveniance. I believe it might be my macbook's network configuration. But I don't know what to do.-

Please help me.-

I look forward to your help!!!