When I go to Mail ---> Preferences ---> Signature , the drop down box titled "Choose Signature" isn't available for me to click (i.e. light colored). And, when composing a new E-mail, the only option in the Signature box is "None".

Why can't I choose the signature that I've made?

 

Any signatures you've set up will be available in the signature list while composing a message.

The "choose signature" selection is to assign a given signature (or method) to a specific mail account. Click an account in the left-most list, and the option will become available.

Pay attention to the "n signatures" beneath each mail account-- if you have 0 signatures for a given account, you will not be able to select one when composing. Click a mail account, then click the plus sign in the next column to create a signature.