Can't seem to find the edit signature link in the USER CP.
I don't think you get a signature unless you've won a medal. Same as the avatars.
As Kurdt mentioned, not everyone can have an avatar or signature. There are three ways to get them though:
1. Contribute to PF. If you wish, you can contribute from this page: https://www.physicsforums.com/payments.php [Broken]
2. Be recognized as a homework helper or science adviser.
3. Be appointed as a part of the Forum Staff (Mentor or Admin).
Separate names with a comma.