Make sure you are on version 1.7.5 of the plugin and that ‘Publish to Discourse’ is selected in the post’s Discourse metabox. Double check for errors in the code you have added. Copying the first code example I provided into your theme’s functions.php file should work in most cases. It can be difficult to debug errors in a live environment. If you have a development version of your site, you can add some code to the wpdc_custom_publish_private_post() function to be sure that it’s being called. I usually create a debug.log file and write to that file to see what’s going on. See this post for details about how to set that up.
If you have added categories to Discourse, you need to select ‘Force Category Update’ from the WP Discourse Publishing options tab and then click ‘Save Options’ to pull in the new categories. You are the second person who has asked about this recently, so I’m going to change that behaviour in the plugin’s next update.