If you want to leave an entry on the discussion board (like say, the caption this contest), scroll down to the bottom of the page with the discussion (NOT this page) to this:
You can enter as a guest if you wish, but if you already have a WFB account (or want to sign up for one because we're awesome), then please sign in before leaving an entry. If near the top of the discussion post you see this:
You are signed in. If you do NOT see that, click on the lock symbol in its place to log in.
When on the caption this topic, to vote on what you think is the funniest and best caption, just click on the ^ to the right of each post:
Note that "guest" is counted as 1 account, so if you can't vote for someone, another "guest" may have already voted for that one. Log into the page as noted above and then you should be able to vote.