Click on "My Active" on the top right of the screen, immediately to the right of your username. (Alternatively, click on the "Your Stuff" dropdown at the top left of the screen and select "Profile". Either of these will get you where you need to be.
Click on "Edit Profile" immediately to the right of your avatar (picture).
Scroll down below the seemingly endless field of checkboxes under "Activities and Interests" -- you should see a large white text box for your "Community Signature."
Fill in your signature information.
Make sure the "I want to show my signature" box is checked.
Click on "Save" at the bottom of the screen.
You should be all set. Note that your current signature will be attached to all of your posts, even the ones you created before you made your signature.
Good luck, and please post back if you have any problems (or if I need to explain things better).