This addon is a plugin. You need to extract it to your /plugins folder to install it.
Nifty syntax highlighter. Add this and you can say that your forum has code written by Google in it!
Caveats: BetterBBCodem, Kses, and Textile don't seem to play nice with code blocks--they either don't play nice at all or consistent enough for this script.
HTML formatter replaces all newlines with <br />s, although it shouldn't touch them inside code or pre blocks. This extension has to do its best to undo this travesty in order to work properly. Thus any <br /> that actually should appear in the code gets turned into a newline.