One of the unique things about Sibelius is that anybody can develop a plug-in for the program using its built-in ManuScript programming language. Over the years, a handful of especially dedicated Sibelius users – chief among them the legendary Bob Zawalich – have developed hundreds of plug-ins for Sibelius, and made them available for users to download and add to their copy of Sibelius absolutely free of charge. At the time of writing, there are just over 370 plug-ins available for download.
The problem is that plug-ins have always been tricky to install: you have to download a zip file, unzip it, put it in the right place, and then restart Sibelius. With the release of Sibelius 7.0.3, this process is now automated by the Install Plug-ins dialog, which you can find in the Plug-ins menu at the bottom of the Backstage view (open a score and click the purple File tab to open Backstage).
To install a plug-in in Sibelius 7.0.3, simply choose File > Plug-ins > Install Plug-ins, and the dialog shown above appears. You can choose between showing only the plug-ins that have been updated or added to the Sibelius web site since you last visited Install Plug-ins or showing all available plug-ins. Expand a category on the left to see all the plug-ins included within, choose the plug-in you want to install, and click Install. Within a moment or two, the plug-in has been downloaded and installed, and is ready to use.
For more details, see 1.22 Working with plug-ins in the updated Sibelius 7 Reference Guide.