Anybody having issues not been able to use mirror image with Mountain lion, the icon does not appear on my MBA 2010 model (late) .
Your mac is out of the spec requirements.
Apparently there is an app called airparrot that works similarly that you could try.
Before anyone chimes in to bitch about apple leaving out older models, have a read of this to realise its not really their fault. The hardware requirements of mirroring mean that the later models handle it with ease while air parrot on the older ones do it, with a lot of compromise.