Hello Paul!
In order to make the Vienna Instruments multitimbral, please use our host plugin Vienna Ensemble. It has been installed together with the Vienna Instruments player.
I'm not sure where you were looking for our instruments in Sibelius. After installing the Vienna Instruments & Vienna Ensemble software, they should be available in Sibelius in the Playback Devices/Available devices list. Can you see Vienna Ensemble there? The latest version of the Vienna Instruments & Vienna Ensemble software can be downloaded here:
http://www.vsl.co.at/en/68/141/460/576/655.htm
If Vienna Ensemble doesn't appear there, I would try to rescan the plugins in the Audio Engine Options menu.
After activating Vienna Ensemble, you can select the sound set in the right column.
More help can be found in the "Optimizing Sibelius Playback" manual and in our tutorial videos:
http://www.vsl.co.at/en/68/141/460/576/655.htm
http://www.vsl.co.at/en/211/1343/1348/243.vsl
Best regards,
Andi
Vienna Symphonic Library