Also see WikidPadGlobals
party out-dated, but maybe still helpful
WikidPad extensions and plugins should be installed in a folder named 'user_extensions', found under the main WikidPad InstallationDirectory?, at the same level as the existing folder 'extensions'. (By default, the installer for Windows uses \Program Files\WikidPad\ as the InstallationDirectory?)
for Linux and Mac, or for running WikidPadFromSource in general, the location should be under the working folder for the wikidpad source files, again at the same level as the folder 'extensions' (if checking out WikidPadSourceCode? via SubVersion? - the UserExtensionFolder? should be in something like ~/svn/wikidpad/work/user_extensions)
Create the directory if it doesn't exist, and copy the contents of the ExtensionsDirectory? into user_extensions before editing.