I was just helping someone out and this came up, now it's an old tip as I have posted about this before but thought would add it again in case anyone new here or anyone else who may not have seen it may find it useful.
Ok so this is all about those categories you add that members can select in your forums, blogs, photos etc ( for instance the little nav we have showing the categories above out forum here Like Critique My Site etc as these are all categories of this forum ). Now these categories normally only show up if there has been a post added to it, which makes sense wright as if no post why show the category, but sometimes you may want them to show up even if nothing is in there. I prefer this as it show the user that the forum or whatever has some options for them to add thier post too.
Ok this is really simple to do.
1. Go to your instance which you want to show them in like a forum
2. Create a post ( or add a photo if photo pages ), it doesn't have to be long just the word test will do for the title and content.
3. Add this to all the categories available
4. Save it and return to main site. Go to that forum and now you will see all the categories show up.
5. Open the test post we added and select edit then delete the post
6. You will now see that even though we deleted the post the categories still show.
Hope you found this useful