This threw me for a loop when I created my LW account. It took some searching around before I found this answer in the solved section of the bug reporting page for the site. In hindsight it seems simple enough, but perhaps it should be added to the wiki page on posting that you need to confirm account via email first.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.