Cant Install On My Air

Hi

i downloaded Lion on my iMac and before installing i copied the dmg file to a usb drive

its installed now and working fine

i am trying to use that file to install on my Air and wont open it

in finder its like a light grey colour

if i go into disk utils and try and say restore and select the image it wont let me but let me select another image for another app

if i double click nothing happens, if i copy it on to the desktop it copies but then its not showing on there

thanks