jEdit plugins can be installed in two ways: as a system plugin or as a user plugin. Below are instructions for installing plugins under UN*X and Win32. If anyone can give me good instructions for any other platforms, email them to me and I'll put them up here as well.

Starting with jEdit 2.2, the system and user plugin directories can always be determined from the Activity Log. On startup, jEdit scans the system and user plugin directories for JAR files to load. Before doing so, it prints the line 'Scanning directory: dir-name' in the Activity Log. The first occurence is the system plugin directory, the second occurence is the user plugin directory.

[ System Plugins ]

UN*X: Extract the plugin in the 'jars' subdirectory of your jEdit source directory and run 'make install'. This will install the new plugin in your $JEDIT_HOME/jars directory.

Win32: Extract the plugin into the 'jars' subdirectory of your installation directory (typically 'C:\Program Files\jEdit\'). The plugin will be loaded next time you run jEdit.

[ User Plugins ]

On all platforms, user plugins should be extracted in the 'jars' subdirectory of your settings directory. This directory can be found in the log messages at the beginning of jEdit's activity log.

macOS: The settings directory by default is in '~/Library/jEdit'.

Windows: The settings directory by default is in '%APPDATA%\jEdit'.

Everything Else: The settings directory by default is in '~/.jedit'.