Adding documents to categories is very simple.
1. Log in to CMS Desk and in Edit mode, select the document from the content tree.
2. Switch to its Properties -> Categories tab.
3. In the categories lists, check the appropriate categories' Select check-boxes and click Save.
Page url: http://devnet.kentico.com/docs/5_5r2/devguide/index.html?adding_a_document_to_a_category.htm