He's attempting to download the new beta of Lightroom (which is available to anyone, but is buggy thus its a beta). When you double-click on the zip file, what should happen is that the package icon should mount, revealing the installer.
I created an account at Adobe.com to download the package myself and see what's going on. I apparently had "open 'safe' files upon downloading" on because Safari automatically unzipped the file and left me with a package icon and ".pkg" file, which is the installer for Lightroom. Double-clicking the .pkg file starts the installer as expected.
The same thing happened when I retrieved the original zip file out of the trash and opened it "manually" by double-clicking on it; the pkg file mounted, I double-clicked that and the installer started. No third-party tool was required to get this (or indeed any file I normally download through web browsers) to unzip.