You can't display comments as a column in calibre, that is a limitation built into calibre, nothing to do with this plugin. You can view comments either on the book details panel or edit metadata view.
If you want to replace the comments for existing books in your library, then at step 2 of the import list wizard you need to select all those matched books, then click on the blue metadata I icon on the right. Select the comments column, then click ok. You should now see the comments data updated on that wizard screen, and completing the wizard will update it officially on those books in your library.
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