How do I use Orchestral Strings in Cubase 7.5?

You simply load Vienna Instruments (VI) as a VST instrument inside Cubase, like you would do with any other sample player, software synth etc.

If you plan to use several instances of VI – which you probably will end up doing – you load Vienna Ensemble (VE) as a VST instrument. VE itself is then used to host all instances of VI.

You could of course also simply open several instances of VI inside Cubase directly, without using VE as the host, but performance-wise it’s better to use VE.