Installing StyleCop for ReSharper under an administrative account does not activate this ReSharper plugin under my developer account.
A system analysis show, that this plugin is installed under
%userprofile%\Local Settings\Application Data\JetBrains\ReSharper\v4.5\vs9.0\Plugins\Microsoft StyleCop for ReSharper
This gives the hint, that the msi must be started for each individual user separately – maybe the is the possibility to move the stuff to all users or that there is also an administrative installation possible.