If you create a blog on our site, you can link it from your profile. Click "Quick Links" in the purple menu bar and then click "Edit Profile". On that page, scroll a little more than halfway down and you'll see the option to link your 3FC blog. Pay close attention to the instructions because if don't enter the right info, the link won't work. Also you can only link a 3FC blog there, not blogs on other sites.
The signature option will automatically appear in your UserCP after you pass your 20 post / 20 days requirement.
Hope that helps