Downloading/Installing Lightroom 4 on new mac

Hi there,

I bought a new computer a couple weeks ago, and just purchased Lightroom 4 for editing photos. I downloaded the program (Lightroom_4_LS11.dmg), but when I click on it, it says that, "The following disc images could not be opened" because it's "not recognized".

How do I download and install Lightroom 4 properly??

I just got a MacBook 10.8.3...

Thanks!

Erin