If the Online Installersays everything is up to date it sounds like Cubase is not scanning the right folder for VST Plugins. You can add the folder manually through the Cubase menu Devices -> Plug-in information.
Look for an option called VST2.x Plugin Paths and add the "C:\Program Files\Steinberg\VstPlugins" folder (or wherever the "Addictive Drums.dll" file is located). After that, restart Cubase and try opening Addictive Drums again.
If that doesn't do the trick, I suggest that you try rescanning VST's in Cubase:
Go to the Cubase menu Devices -> Plug-in information, then click the "Update" button. Does that work? Is Addictive Drums in the Plug-in information list?
If this does not solve anything, please contact our support department at www.xlnaudio.com/support