I had to unlock my "System" keychain (right click the keychain in the left bar and select "unlock Keychain 'System'"). Then I double clicked the certificate in Finder. It installed with no problems at that point.

Was this appeal effective? I've appealed a 4.3.0 Spam and keep getting rejected despite offering a unique app with unique content. I'm really stuck trying to figure out what I need to change in order to shed the Spam label. I also have updates that I'm pretty desperate to push out to my users.