Why doesn't unity 4.1.5 want to install on OSx 10.8.3?

Well…
I’m currently on my mac, and want to install Unity 3D.

When opening the DMG it says after verification “invalid checksum”
so I skip the verification, and during install I get the error “the installer encountered an error that caused the installation to fail”

What is wrong?!
It’s unity 4.1.5
my time and dates are correctly on my mac,
and I don’t get it what’s wrong!

I reformatted (deleted my HDD, formatted, and reinstalled 10.8.3)
Downloaded again, and gets the same error!
Did “Repair disk” on disk utilities, and even partitioned the HDD to try and install on a different partition, it still didn’t work…
What can I do?

http://forum.unity3d.com/threads/185382-4-1-5-DMG-for-Mac-is-corrupted