Hello Timo!
Please make sure that the sound set files (xml files) are copied to the correct location. The correct location depends on your operating system and Sibelius version. For Sibelius 5 these files have to be copied to one of the following folders:
Windows Vista, Windows 7 und Windows 8:
Users\your username\AppData\Roaming\Sibelius Software\Sibelius 5\Sounds
Windows XP:
Documents and Settings\your username\Application Data\Sibelius Software\Sibelius 5\Sounds
Mac OS X:
Users/your username/Library/Application Support/Sibelius Software/Sibelius 5/Sounds
Under Mac OS X 10.7 and 10.8 the Users/your username/Library folder is hidden by default. To get access go to the folder Users/your username. Then choose Go/Go to folder and enter “Library”.
If you are working on a Windows computer and Vienna Instruments & Vienna Ensemble do not appear in the list of available devices, you will have to “tell” Sibelius the path where your virtual instruments and effects are located. To do so, go to Playback Devices/Audio Engine Options/Virtual Instruments and Effects Folderand choose the path where you installed the Vienna Ensemble VST plugin (= the path where the file Vienna Ensemble.dll is located). Then restart Sibelius.
Best regards,
Andi