Many thanks for raising the subject.
I've never used them before, but the way you appear to add them is via a dialogue box, just above the "quick reply" area at the bottom of your posting. Over on the right of the box, there's an "edit tags" link. Click on that and it enabled me to add 2 tags to your post.
To retrieve tagged items, you can click on the "tags" link in the "what's happening table on the left of the screen.
Hope this helps.
PS You can also retrieve postings related to the tags concerned by just clicking on the approrpiate tag attached to the posting.