You don't add the article directly to the category. In the search field, type in the name of your article, and click "create this page" when the search provides no results. Then, from another nation profile page currently in use, copy and paste the nation template coding and fill in the fields with the information for your micronation. Save the page and it will be automatically added to the nation category (as the template is programmed to add it to that category).
For ease, I suggest you use the following link to get the code for the nation template (the column of data at the start of the text box):
http://mnn.mncentre.net/wiki/index.php?title=Rocentia&action=editCopy everything between and including the {{ and }}