Not quite sure what you're doing in regards to webmatrix, to be honest. Logically, as long as they can access the website somehow, and interact with it and log in, they should be able to earn the achievements. My suggestion from earlier was to help get the single achievement url working, as i perceived you having an issue and it showing 404 errors instead. Visiting the permalinks page in your WP Admin is a way to trigger a rewrite rules flush/rebuild and get any new post types working as expected.
You would just have them visit the url in question and fill out the forum.