Installation errors for LR4 using disc on iMac OX X Lion

I'm trying to install an upgrade of LR4 on my iMac running OS X Lion 10.7.5 from a disc.

Every time I try to install I get the following error:

"The Installer encountered an error that caused the installation to fail. Contact the software manufacturer for assistance."

My console says:

11/8/12 5:20:19.129 PM authorizationhost: SFBuiltinEntitled: Installer.app is not entitled for system.install.app-store-software

11/8/12 5:20:19.129 PM com.apple.SecurityServer: Failed to authorize right 'system.install.app-store-software' by client '/System/Library/PrivateFrameworks/PackageKit.framework/Versions/A/Resources/installd' [371] for authorization created by '/System/Library/CoreServices/Installer.app' [364]

11/8/12 5:20:19.131 PM installd: PackageKit: ----- Begin install -----

11/8/12 5:20:19.181 PM installd: PackageKit: Install Failed: Error Domain=PKInstallErrorDomain Code=102 "The package “Adobe Photoshop Lightroom 4.pkg” is untrusted." UserInfo=0x10d3310e0 {NSLocalizedDescription=The package “Adobe Photoshop Lightroom 4.pkg” is untrusted.,

...there's  more but this is the basic gist of it.

I noticed one other mention of this error with similar info from their console in the forums, but the problem was that the user needed to upgrade to 10.7.5 which I have. I have no idea where to go next. Any help would be much appreciated