No particular folder is required for the install of Fireworks symbols, so you can install the PNG images in any location. The most convenient locations are probably going to be your folder in your user account as you will have full access to save and overwrite the files. To save them to the program files / software folder / applications you will need administrator privilege (which is not an issue if you are using Fireworks on a single user machine).
Startup Fireworks and go to the window menu command. Load the PNG files via the symbols import libraries panel. Opening the PNG document may or may not display the art as some of the earlier sets did not include the them on the artboard. All the most recent sets include the artworks in the panel and on the document for ease of editing and modification
Fireworks select PNG image
Select the items you want from the library - personally I wish the app just selected them all by default on selecting the PNG image