Please, don't bump same questions. Ask about it on the plugin's page, or even on GitHub's repo
The longtext input fields will already use ckeditor as long as you have the ckeditor plugin that comes with Elgg enabled.
The ckeditor_addon plugin is to be used in addition to the bundled ckeditor plugin. Once you have the ckeditor plugin enabled, you can configure the editor options of ckeditor in the plugin settings of the ckeditor_addon plugin. Nothing else to do.
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