I could solve the Problem on my side - updating Outlook 2007 to 2010 made it possible to activate the Add-in.
On this machine was a Trial Version of Office 2010 installed, as we bought it and we deinstalled this and installed Office 2007 because of the policy in this Company. Maybe the preinstallation of 2010 would have caused the issue? - because all of my other Trials to solve the problem were not successful.