My set up is: Sibelius 4.1 - GPO - GPO crossgrade.
When choosing "modern orchestra" as the basic sound set, and following the method to impose pizz on strings (i.e. text -> technique -> pizz. written where pizz is supposed to begin) - no pizzicato results. I thought these kinds of effects were built in such that they need only be invoked as above in order to work.
Re: No pizzicato for strings in "modern orchestra"
Try writing the pizz without the period - I think I had to do this once. Another thought: make sure that the technique text is attached to the correct staff - it's easy for them to become seperated from their intended location. If that fails, try posting this on the Sibelius board.