You can access it through going to your "profile", and then going to your "forum profile information" which is a sub category on the left hand side of the page. Then navigate to the signature sub header. There are limitations depending on the rank of the user, you can check these out here:
https://bitcointalk.org/index.php?topic=2766177.0and here:
https://bitcointalk.org/index.php?topic=178608.0In future, anything relating to the forum will be included in the Meta section (
https://bitcointalk.org/index.php?board=24.0), and there's very likely a stickied thread for the most common of issues.