That's because you have not yet been a member for long enough. After you've been a member for 90 days (around 3 months) you'll see an area for adding/editing a signature. Some time in the next month you should be able to do that (depending on when you signed up.)
The reason for this is to discourage people from registering just so they can start to spread signature links everywhere. Anyhow, even when you can, the signature links are no-follow, so they are useless in terms of SEO. They are only of any use if you post regularly, so that other members see your links on a regular basis.