I can't install photoshop on my mac. It says my operative system does not support it, even though I had cs5 on the same mac a few days ago.

I deleted my Photoshop cs6 to install it again with a different serial number, and when I tried to download it again it said that my operative system didn't support it.
I have the newest update, and Lion 10.7.5, what can I do? I really need it....