Source: Jackett/Jackett#1576 (comment)
From the Jackett page, click the "add indexer" button so that the pop up window with the full list of indexers appears.
You'll then need to open your browser's development toolbar (in Chrome just hit F12) and go to the JavaScript Console and enter the following:
////hack to add all free indexers in Jackett
$(document).ready(function () {
EnableAllUnconfiguredIndexersList();