I think you should be able to use it already - by clicking the User CP button in the upper left corner, you can access all of your account settings, and Edit Signature is a coupla notches down on the menu under the heading of Settings & Options. If it isn't working, let us know.
I think I don't have the "Edit Signature" button. Under "Settings & Options" I can find only these four buttons: "Edit Avatar", "Edit Email & Password", "Edit Options", and "Edit Ignore List". Looked for the "Edit Signature" button elsewhere too on the "User CP" page but could not find it.
No, it isn't compulsory. Many of us list our meds & management methods, which makes it easier in discussions when we can see all the things that might have some bearing on the ups/downs of our management.
Some of you have a nice signature/profile at that bottom of each post, such as Shanny. How do you enable that type of information to be shown in each post. The standard signature is just text without the nice graphics/formatting.