The easiest way to find a plugin or to find the category of a plugin is to use the list on Philip Rothman's Scoring Notes blog, at https://www.nycmusicservices.com/musicresources/#tab-3-4-content. Click on the link to Sibelius Plugins and use the Search box to find the plugin you want. The search results will show you the category, and the name itself is a link to the entry for that plugin on the download page, which saves you needing to know the category if you are going to download and unzip the file yourself.
For versions later than Sib 7, follow the instructions up to the point where they say
(Windows) “You should see a folder called Sibelius 7, Sibelius 6, Sibelius 5 or Sibelius 4: if so, double-click it to go inside it; if not, create one, then go inside it”
(Mac) “You should see a folder called Sibelius 7: if so, double-click it to go inside it; if not, create one, then go inside it.”
If you are using Sibelius 7.5, replace “Sibelius 7” with “Sibelius 7.5”. If you are using Sibelius 8.x, replace “Sibelius 7” with “Sibelius” (no number). Then continue with the instructions.
Be sure that you close and restart Sibelius once you have put the .plg file into the correct folder. Plugins are loaded into memory only when Sibelius starts up, or when they are installed with File > Plug-ins > Install Plug-ins.
The downloadable plugin Install New Plugin (category Other) can make the process a little less painful. You will still need to download and unzip the plugin from the download page, but Install New Plugin will give you a list of available plugin folders so you don’t have to remember where they are. Of course you will need to install "Install New Plugin" itself, but that is something you only need to do once.
Note that if you install plugins manually, you may find that shortcuts assigned to plugins will bring up the wrong plugin. To correctly reset the shortcuts go to File - Preferences - Keyboard Shortcuts and click on OK.