Make sure that you can attach a file manually before trying to use DocXtender on a new page.
The Business Central page must have a “Document Attachment FactBox” pointing at a valid single record on any page you want to use DocXtender. The Attachment or Link factbox (not just the tab) must also be visible, so you may need to open personalization to add it. To add attachments to any Business Central page that does not include the factbox, see: http://kb.dmsiworks.com/knowledge-base/extend-for-drag-and-drop-document-attachments-to-any-table-and-page/ (this only adds the drag & drop function – you still need to add the attachment function to the page first).
Note that Journals use the Document Attachment Overview table (137) instead of the Document Attachment table (1173), so they are not supported by DocXtender.